In Situ Treatment Technologies for 

Contaminated Soil 

ENGINEERING FORUM ISSUE PAPER

Contents 

Section Page 

1.0 Introduction ......................... 1 

2.0 Background on Issue Paper ............. 2 

3.0 In Situ Treatment Technologies ......... 2 

3.1 Physical/Chemical Treatment 

Technologies ..................... 2 

3.1.1 Soil Vapor Extraction ......... 2 

3.1.2 Solidification/Stabilization ..... 6 

3.1.3 Chemical Oxidation ........... 9 

3.1.4 Soil Flushing . . . . . . . . . . . . . . . 12

3.1.5 Electrokinetic Separation . . . . . . 14 

3.2 Biological Treatment Technologies . . . 15 

3.2.1 Bioventing . . . . . . . . . . . . . . . . . 16

3.2.2 Phytoremediation . . . . . . . . . . . . 20

3.2.3 Monitored Natural Attenuation . 23 

3.3 Thermal Treatment Technologies . . . . 25 

3.3.1 Electrical Resistance Heating . . . 26 

3.3.2 Steam Injection and Extraction . 26 

3.3.3 Conductive Heating . . . . . . . . . . 29

3.3.4 Radio-Frequency Heating . . . . . 31 

3.3.5 In Situ Vitrification . . . . . . . . . . 32

4.0 Notice, Disclaimer, and Acknowledgments 34 

5.0 Abbreviations and Acronyms ........... 35

1.0 Introduction 

This issue paper provides summary information on 

a wide variety of in situ technologies for the 

treatment of contaminated soil in both the vadose 

zone and saturated and unsaturated source zones. 

The in situ technologies presented involve applying 

chemical, biological, or physical processes to the 

subsurface to degrade, remove, or immobilize 

contaminants without removing the bulk soil. 

Compared to excavation and ex situ treatment, the 

use of these technologies offers several benefits, 

such as addressing deep contamination and generally costing less. 

The summary for each in situ technology includes a 

basic description of the technology, its implementation, applicability based on contaminants and site 

characteristics, general limitations, costs, and status 

of the technology’s application. Information in this 

paper is intended to give project managers and engineers a basic understanding of the technology that 

will allow for further consideration of its applicability at a site. Project managers and engineers 

seeking guidance on the design and operation of 

these technologies should refer to the references 

listed in this paper and other material on the specific 

technology of interest. 

The treatment technologies presented include 

common practices as well as innovative alternatives 

for treating contaminated soil and source zones in 

situ. The paper does not address technologies in the 

experimental phase, such as nanoscale iron injection, nor does it present containment technologies, 

such as capping, liners, and barrier walls. 

Information provided in this paper comes from a 

number of sources. In general, every attempt has 

been made to use technical literature, including 

articles, textbooks, and U.S. Environmental Protection Agency (EPA) and other agency documents. 

Where appropriate and possible, Web links have 

been provided for additional information. This paper 

is not intended to serve as guidance or policy, nor 

does it indicate the appropriateness of using a 

technology at a specific site. 

United States Solid Waste and EPA 542/F-06/013 

Environmental Emergency Response November 2006 

Protection Agency 5203P www.epa.gov/tio/tsp 
2.0 Background on Issue Paper 

This issue paper was developed at the request of 

EPA’s Engineering Forum to provide information to 

EPA project managers on the application of in situ 

treatment technologies for contaminated soil. The 

Engineering, Federal Facilities, and Ground Water 

Forums, established by EPA professionals in the ten 

regional offices, are committed to identifying and 

resolving scientific, technical, and engineering issues 

impacting the remediation of Superfund sites and 

corrective action sites under the Resource Conservation and Recovery Act (RCRA). The forums are 

supported by and advise the Office of Solid Waste 

and Emergency Response’s (OSWER) Technical 

Support Project, which established Technical Support 

Centers in laboratories operated by the Office of 

Research and Development, Office of Radiation Programs, and the Environmental Response Team. The 

centers work closely with the forums, providing stateof-the-science technical assistance to EPA project 

managers. 

3.0 In Situ Treatment Technologies 

For purposes of this paper, the in situ technologies are 

categorized into three major groups based on the 

primary mechanism by which treatment is achieved: 

• Physical/Chemical Treatment Technologies 

• Biological Treatment Technologies 

• Thermal Treatment Technologies 

Physical/chemical treatment includes soil vapor extraction, solidification/stabilization, soil flushing, 

chemical oxidation, and electrokinetic separation. 

Biological treatment uses microorganisms or vegetation to degrade, remove, or immobilize contamination 

in soil. Biological technologies include bioventing, 

phytoremediation, and monitored natural attenuation. 

Electrical resistivity heating, steam injection and extraction, conductive heating, radio-frequency heating, 

and vitrification are technologies summarized under 

thermal treatment. Table 1 provides a general summary of the effectiveness of the technologies for 

various contaminant classes. 

The principal feature of many in situ treatment 

technologies is delivery and recovery of fluids or 

other reactants to the subsurface. The ability to control and monitor the delivery and recovery of these 

fluids or reactants is central to the effectiveness of in 

situ technologies in treating the contamination. 

Depending on the subsurface conditions and contaminant characteristics, each in situ technology has 

benefits and limitations on its ability to effectively 

deliver, control, and recover administered fluids 

and/or reactants and the contaminants. For example, 

soil permeability is an important factor in the delivery of a reactant for chemical oxidation or a gas 

for bioventing, whereas it is not as important for 

conductive heating. Consequently, the characterization of this parameter would generally be more 

critical for chemical oxidation or bioventing than for 

conductive heating. 

The increased use in recent years of several in situ 

soil treatment technologies, such as chemical oxidation and thermal treatment, has shown that both 

technologies are a viable option for addressing 

source zones contaminated by nonaqueous phase 

liquids (NAPLs). In addition, greater emphasis is 

being placed on examining these technologies for 

their potential synergies as treatment trains to address contamination in the subsurface. This integrated approach has the potential for providing a 

more effective site remediation. 

For information on various in situ technologies: 

Harzardous Waste Cleanup Information (CLU-IN) 

website at: http://www.cluin.org/techfocus/ 

Federal Remediation Technologies Roundtable 

(FRTR) website at: http://www.frtr.gov/ 

Naval Engineering Facilities Environmental Restoration & BRAC (NAVFAC) website at: http:// 

enviro.nfesc.navy.mil/erb 

3.1 Physical/Chemical Treatment Technologies 

Physical/chemical technologies, which represent the 

most diverse group of remediation technologies, include soil vapor extraction, solidification/stabilization, oxidation, soil flushing, and electrokinetic 

separation. 

3.1.1 Soil Vapor Extraction 

In situ soil vapor extraction (SVE) is a remediation 

technology in which a vacuum is applied to induce 

a controlled subsurface air flow to remove volatile 

organic compounds (VOCs) and some semivolatile 

organic compounds (SVOCs) from the vadose zone 

2 
Table 1. Summary of In Situ Treatment Technologies Applications for Contaminant Classes 

Key:  Better TM Average  Worse 

S=Specific to chemical type 

Nonhalogenated VOCsHalogenated VOCsNonhalogenated SVOCsHalogenated SVOCsFuelsInorganicsRadionuclides1 

Explosives 

In Situ Physical/Chemical 

Soil Vapor Extraction        

Solidification/Stabilization   TM TM    

Chemical Oxidation TM TM  TM  S  TM

Soil Flushing   TM TM TM   

Electrokinetic Separation TM TM TM TM   TM 

In Situ Biological Treatment 

Bioremediation    S  S S 

Bioventing        

Phytoremediation TM TM TM S TM TM  

In Situ Thermal 

Thermal Treatment (electrical resistivity heating, steam 

injection and extraction. conductive heating, radiofrequency 

heating, and in situ vitrification) 

       

Adapted from Federal Remediation Technologies Roundtable Remediation Screening Matrix, Table 3.2. 

http://www.frtr.gov/matrix2/section3/table3_2.html 1

For more information on radionuclide technologies see: U.S. EPA.1996. Technology Screening Guide for 

Radioactively Contaminated Sites, EPA/402/R-96/017. 

http://www.epa.gov/superfund/resources/radiation/pdf/techguide.pdf 

to the surface for treatment. The configuration of the 

system usually involves attaching blowers to extraction wells which are generally constructed with 

slotted polyvinyl chloride (PVC) to induce airflow 

through the soil matrix (Army Corps of Engineers 

[USACE] 2002). The contaminated air is brought to 

the surface and passed through a vapor/liquid 

separator to remove any moisture before the air is 

treated. Treatment is typically done by adsorption 

(activated carbon), or for more concentrated waste 

streams, by thermal oxidation systems (U.S. EPA 

2006). The water generated by the liquid separator 

may also require treatment (Figure 1). When expected 

concentrations in the air stream are sufficiently high 

(1,000 to 5,000 parts per million [ppm] or more) for 

free product recovery for recycling, a stand alone 

condensation treatment system might be considered. 

This type of system is generally not used for 

mixtures of chemicals, and at some point the 

condenser system will need to be changed out when 

concentrations drop (USACE 2002). 

Concrete, asphalt, geomembrane, or other lowpermeability covers are often placed over the soil 

surface to prevent short-circuiting of air flow and to 

increase the radius of influence of the extraction 

wells. Replacement air can be introduced into the 

subsurface by injecting air via a blower or by allowing air to flow into passive injection wells. 

While vertical wells are the most widely used SVE 

design method, when the contamination and/or the 

water table is shallow, horizontal wells or trenches 

provide better lateral flow and superior formation 

access. 

3 
The SVE process is driven by the partitioning of 

volatile materials from condensed phases (sorbed on 

soil particles, dissolved in pore water, or nonaqueous 

liquid) into the soil gas being drawn through the subsurface. The partitioning is controlled by contaminant and soil properties. These properties include 

contaminant vapor pressure, Henry’s law constant, solubility, soil intrinsic permeability, water content (which 

should be low, but very 

dry soils also inhibit 

contaminant mobilization), and organic carbon content (Air Force 

Center for Environmental Excellence 

[AFCEE ] 2002). SVE is 

best suited in welldrained, high-permeability soil (sand and 

gravel) with a low organic carbon content. 

Low permeability soil or 

heterogenous soil with 

high carbon content are 

more difficult to treat 

with SVE and often 

require amendments, 

such as pneumatic or 

hydraulic fracturing.1 

Fracturing allows for 

high preferential flow Figure 1. Typical Soil Vapor Extraction System 

After a suitable (site-specific) time, the blowers are 

turned back on to capture the more concentrated soil 

vapors (AFCEE 2002). If appropriate, this method 

can save money on electricity and other costs. For 

other examples of energy conservation, see Gill and 

Mahutova (2004). 

paths, but the bulk of the 

contaminant load still depends upon low flow or 

diffusion from the competent soil matrix. 

Like fracturing, heterogenous subsurfaces provide 

differential flow paths that result in efficient removal 

of contaminants in the permeable layers, with the less 

permeable layers being subject to slow diffusive 

forces. Rate-limited diffusion in the less permeable 

soils extends the time needed for remediation; 

therefore, it may be more efficient to approach these 

types of sites with a pulsed pumping strategy, in 

which the blowers are turned off at predetermined 

effluent concentrations, and the contaminants are 

allowed to diffuse into the “clean” permeable layers. 

1 Fracturing is the creation of cracks or sand-filled fissures in 

low- permeability formations. 

When designing an SVE system, DiGiulio and 

Varadhan (2001) advise care in choosing standard 

radius of influence (ROI) methods to place extraction wells. These methods generally rely on measuring vacuum differentials with distance from the 

venting well. Vacuum measurements can indicate 

the direction of a flow gradient, but as the vacuum 

measured approaches ambient pressures, they may 

give a false indication and lead to placing wells too 

far apart. In addition, vacuum measurements give no 

information on the effective gas flow through the 

various subsurface materials. For example, one-dimensional measurements made on layers of sand and 

silty clay will yield equivalent vacuums, while the 

effective gas flow is through the sand, with little 

going through the silty clay. A more relevant approach to well layout is to achieve a pore velocity 

4 
that exceeds some minimum rate everywhere within 

the contaminated zone (USACE 2002). 

As the vapor extraction system continues to operate, 

effluent contaminant concentrations generally become asymptotic (steady-state removal of very low 

concentrations). Unless the SVE system is addressing 

a single contaminant species, measurements of the 

venting effluent should provide the total mass being 

removed as well as relative compound concentrations. Speciation data also help in evaluating the 

system’s efficiency. Because the chemicals in a mixture have different chemical/physical properties, they 

will leave the mixture at different rates; hence, a drop 

in total concentration does not necessarily mean a 

drop in available contaminant or system efficiency, 

but rather exhaustion of certain species. It is also 

important to test each extraction well in the system 

individually to determine if the drop is occurring 

across all wells (USACE 2002). Testing of the header 

alone may mask wells that have low flow and high 

concentrations that are being diluted by other wells in 

the system. 

Maintaining asymptotic levels over a period of many 

months is often interpreted as a sign that the SVE 

effort has been successful and should be shut down; 

however, as USACE (2002) states: “although the 

decrease of concentrations in the extracted vapor is an 

indication of the effectiveness of the system, it is 

certainly not conclusive evidence that the concentrations in the soil have decreased proportionally.” 

Reasons for a decrease in contaminant concentration, 

other than reaching cleanup goals, include: 

• The system has exhausted the supply of contaminants that it can advectively reach, and their 

continued presence, at very low concentrations, 

represents a draw upon diffusion rate-limited 

source areas. 

• The water table has risen and the source areas are 

no longer available to the SVE system. 

• The soil has reached a dryness factor that hinders, 

rather than promotes, SVE. 

• The measured flow represents dilution from fully 

flushed areas near the extraction well, while 

understating considerably more contaminated 

areas further away, near stagnation points 

(AFCEE 2002 and DiGiulio and Varadhan 2001). 

If no rebound is found after shutting the system 

down for a site-specific determined time, then 

confirmation sampling should be done. Confirmation sampling can be accomplished with an 

extensive soil gas survey, continuous soil sampling 

on a statistically determined grid, or professional 

judgment with sufficient previous characterization 

information gained by use of direct push tools, such 

as the membrane interface probe or, in the presence 

of hydrocarbons, by laser-induced fluorescence 

spectroscopy. 

If a site has contaminated groundwater, it should be 

addressed along with the vadose zone contamination. Often this can be accomplished using a 

multi-phase extraction (MPE) system to simultaneously remove contaminants from soil and extract 

contaminated groundwater. A discussion of MPE, 

which is not within the scope of this document, can 

be found in U.S. EPA (1999) and USACE (1999). 

The cost of SVE is site-specific and depends in part 

on the hydrogeology, type and amount of contaminants, and whether the offgas requires treatment. 

The FRTR website estimates the cost is between $10 

and $40 per cubic yard, with a typical pilot program 

costing between $10,000 and $40,000. The 

NAVFAC website provides a $20 to $60 per cubic 

yard estimate. USACE (2002) provides a strategy 

for estimating costs and a checklist for items to 

include in the estimate. SVE is a mature, widely 

used technology, and many vendors are capable of 

implementing the technology. 

Cited and Other References 

AFCEE. 2000. Source Reduction Effectiveness at 

Fuel-Contaminated Sites: Technical Summary Report. 125 pp. http://www.afcee.brooks.af.mil/pro 

ducts/techtrans/download/SourceRed.pdf 

American Academy of Environmental Engineers. 

1998. Innovative Site Remediation Technology 

Design & Application, Volume 7: Vacuum Extraction and Air Sparging, EPA 542/B-97/010. U.S. 

EPA, Office of Solid Waste and Emergency Response, 392 pp. http://nepis.epa.gov/pubtitleOSW 

ER.htm 

DiGiulio, D. and R. Varadhan. 2001. Development 

of Recommendations and Methods to Support 

Assessment of Soil Venting Performance and 

Closure, EPA 600/R-01/070. U.S. EPA Office of 

5 
Research and Development, 435 pp. http://www.epa. 

gov/ada/pubs/reports.html 

FRTR website at: http://www.frtr.gov/ 

Gill, M. and K. Mahutova. 2004. Engineering Forum 

Issue Paper: Introduction to Energy Conservation and 

Production at Waste Cleanup Sites, EPA 542/ 

S-04/001. U.S. EPA, Office of Solid Waste and 

Emergency Response, 36 pp. http://www.epa.gov/ 

tio/tsp/download/epa542s04001.pdf 

Johnson, R., R. Dupont, and D. Graves. 1996. 

Assessing UST Corrective Action Technologies: 

Diagnostic Evaluation of In Situ SVE-Based System 

Performance, EPA 600/R-96/041. U.S. EPA, Office 

of Research and Development, 162 pp. http://nepis. 

epa.gov/pubtitleORD.htm 

NAVFAC. 2006. Naval Engineering Facilities 

Environmental Restoration & BRAC website. 

https://portal.navfac.navy.mil/portal/page?_pageid= 

181,5346904&_dad=portal&_schema=PORTAL 

USACE. 1999. Engineering and Design: Multi-Phase 

Extraction, EM 1110-1-4010, 286 pp. http://www. 

usace.army.mil/publications/eng-manuals/em1110­

1-4010/toc.htm 

USACE. 2002. Engineering and Design: Soil Vapor 

Extraction and Bioventing, EM 1110-1-4001, 424 pp. 

http://www.usace.army.mil/inet/usace-docs/engmanuals/em1110-1-4001/toc.htm 

USACE. 2006. Army Corps of Engineers Environmental webpage. http://www.environmental.usace. 

army.mil/ 

U.S. EPA. 1997a. Analysis of Selected Enhancements for Soil Vapor Extraction, EPA 542/R-97/007. 

Office of Solid Waste and Emergency Response, 246 

pp. http://www.cluin.org/download/remed/sveen 

hmt.pdf 

U.S. EPA. 1997b. Michigan Soil Vapor Extraction 

Remediation (MISER) Model: A Computer Program 

to Model Soil Vapor Extraction and Bioventing of 

Organic Chemicals in Unsaturated Geological 

Material, EPA 600/R-97/099. Office of Research and 

Development, 260 pp. http://nepis.epa.gov/pubtitle 

ORD.htm 

U.S. EPA. 1999. Multi-Phase Extraction: State-ofthe-Practice, EPA 542/R-99/004. Office of Solid 

Waste and Emergency Response, 78 pp. http:// 

www.epa.gov/tio/download/remed/mpe2.pdf 

U.S. EPA. 2006. Off-Gas Treatment for Soil Vapor 

Extraction Systems: State of the Practice, EPA 

542/R-05/028. Office of Solid Waste and 

Emergency Response, 129 pp. http://www.cluin. 

org/download/remed/EPA542R05028.pdf 

3.1.2 Solidification/Stabilization 

Solidification and stabilization (S/S) refer to closely 

related technologies that use chemical and/or 

physical processes to treat radioactive, hazardous, 

and mixed wastes. Solidification technologies encapsulate the waste to form a solid material. The 

product of solidification may be a monolithic block, 

a clay-like material, a granular particulate, or some 

other physical form commonly considered “solid.” 

Stabilization technologies reduce the hazard potential of a waste by converting the contaminants into 

less soluble, mobile, or toxic forms (e.g., Cr(VI) to 

Cr(III)). The physical nature and handling characteristics of the waste are not necessarily changed 

by stabilization. 

Chemical stabilization relies on the reduction of 

contaminant mobility by physical or chemical reactions with the contaminant, rather than the contaminant matrix (e.g., soil or sediment), as is done with 

solidification. The mobility of organic and inorganic 

compounds can be reduced through various precipitation, complexation, and adsorption reactions. 

Commonly applied inorganic stabilization agents 

include soluble silicates, carbon, phosphates (e.g., 

apatite), and sulfur-based binders. Organo-clays 

have been used to stabilize organic chemicals that 

are poorly addressed by precipitation and complexation reactions (U.S. EPA 1997). 

The S/S process can be accomplished using either 

inorganic or polymer binders. The most common 

inorganic binders are Portland cement, pozzolans 

(siliceous or aluminous materials that can react with 

calcium hydroxide to form compounds with cementitious properties), and cement/pozzolan mixtures. 

While these binders are effective for a range of 

inorganic cations and anions, a treatability study 

6 
should be conducted using on-site soil, contaminants, and groundwater (if applicable). 

In situ chemical stabilization of inorganics using 

phosphorus based and other compounds was 

evaluated in September 1998 under EPA’s Superfund 

Innovative Technology Evaluation Program (SITE). 

The Soil Rescue and EnvirobondTM remediation 

products were applied to a small area of leadcontaminated soil at the Crooksville/Roseville Pottery 

site in southeastern Ohio. These products chelate the 

metal ions to reduce mobility. The mean Toxicity 

Characteristic Leaching Procedure (TCLP) lead 

concentrations were reduced by more than 99 percent 

for both products (U.S. EPA 2002 and 2003). 

S/S treatment of organic contaminants with 

cementitious formulations is more complex than 

treatment of inorganic contaminants. While low 

levels of organic contaminants can 

be treated using S/S, many organics 

will interfere with the hydration 

process and impede the curing of 

the solid (U.S. EPA 1997). 

Subsurface variations in the concentrations of organics can affect 

both the leachability and final 

physical properties of the treated 

wastes or soil. Thorburg et al. 

(2005) used Portland cement to 

treat a sediment contaminated with 

coal tar-derived hydrocarbons. The 

results showed that the treated 

sediments leached polycyclic aromatic hydrocarbons (PAHs) and 

midrange aromatic and aliphatic 

hydrocarbons at concentrations 

Vertical auger mixing requires a system of augers to 

inject and mix binder into the soil (Figure 2). The 

treatment depth is limited by the torque required to 

turn the auger. Current testing indicates a limit of 

depths to less than 150 feet. The auger diameter, 

which determines the number of holes that need to 

be drilled for a given areal extent, can range from 

several meters for shallow mixing to much smaller 

diameters for deep mixing. The need for a smaller 

diameter auger means more holes will need to be 

drilled per unit area, which increases the cost for the 

deeper mixing. If VOCs or mercury are present at 

the site, the contaminant vapors should be captured 

and treated. The capture is usually accomplished 

with a hood that covers the mixing area and conveys 

the gases to an on-site treatment system. Auger 

mixing is the most commonly applied method for in 

situ mixing of S/S reagents with soil. 

well above their effective solu- Figure 2. MecToolTM for Solidification and Stabilization of 

bilities. Most cementitious pro- Contaminated Soils and Sludges 

cesses are exothermic, and the heat 

generated by the curing process has the potential to 

volatilize VOCs. 

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. Three basic approaches are used for mixing 

the binder with the matrix: 

• Vertical auger mixing 

• Shallow in-place mixing 

• Injection grouting 

In-place mixing involves the spreading and mixing 

of binder reagents with waste by conventional earthmoving equipment, such as draglines, backhoes, or 

clamshell buckets. A large auger rig can also be 

employed for in-place mixing. The technology is 

applicable only to surface or shallow deposits of 

contamination. 

A novel form of in-place waste mixing can be used 

for large areas of heavy-metals contaminated soil. A 

lime-stabilized biosolid can be plowed into the 

contaminated soil, yielding a mixture that reduces 

toxicity and bioavailability of the heavy metals 

7 
while providing a soil suitable for supporting 

vegetation. 

Injection grouting involves forcing a binder containing dissolved or suspended treatment agents into 

the formation under pressure, thereby permeating the 

soil. Grout injection may be applied to contaminated 

formations lying well below the ground surface. The 

injected grout cures in place, producing an in situ 

treated mass. 

Polymer binders are thermoplastic or thermosetting. 

Thermoplastic binders are materials that can be repeatedly melted to a flow state and will harden when 

cooled. Polyethylene, sulfur polymer, and bitumen 

are examples of theromoplastic binders. Thermosetting binders are materials that require the combination of several liquid ingredients (e.g., monomer, 

catalyst, promoter) that, when combined, harden to a 

solid that cannot be reworked (U.S. EPA 1997). 

Thermoplastic binders operate in a temperature range 

of 120 to 180o

C, which could be an issue in soil with 

high moisture content. Thermosetting binders operate 

at ambient temperatures, but they are not amenable to 

high moisture content. While polymer binders are 

effective, they may be difficult to use in an in situ 

setting. 

S/S has been applied to the remediation of hazardous 

waste sites for more than 15 years. Experience with 

the technology, especially the inorganic binders 

(Portland cement and pozzolans), is abundant. 

The Army Environmental Policy Institute (1998) 

estimates that in situ S/S of metals using a phosphoric apatite binder costs approximately $46 per ton; 

using Portland cement for metals costs about $125 

per ton; using ammonium modified Portland cement 

for organics costs about $101 per ton; and using 

polyethylene costs about $609 per ton. The Portland 

Cement Association also has costing data: 

http://www.cement.org/. 

Cited and Other References 

Army Environmental Policy Institute. 1998. 

Solidification Technologies for Restoration of Sites 

Contaminated with Hazardous Wastes, 80 pp. 

http://www.aepi.army.mil/pubs-cleanup.html 

Maher, A., H. Najm, and M. Boile. 2005. Solidification/Stabilization of Soft River Sediments Using 

Deep Soil Mixing. State of New Jersey Department 

of Transportation, 44 pp. http://www.state.nj.us/ 

transportation/works/maritime/documents/deep 

soilmixingfinal.pdf 

Portland Cement Association. 2006 (November). 

Portland Cement Association webpage. http://www. 

cement.org/waste/wt_tech_epapubs.asp 

Thornburg, T. et al. 2005. Effectiveness of in situ 

cement stabilization for remediation of sediment 

containing coal tar derived hydrocarbons. The Annual International Conference on Contaminated 

Soils, Sediments, and Water, October 17-20, 2005, 

University of Massachusetts, Amherst. http://www. 

umasssoils.com/abstracts2005/Thursday/evolving 

%20strategies.htm 

USACE. 2006. Unified Facilities Guide Specifications: Section 02 55 00, Solidification/Stabilization 

(S/S) of Contaminated Material, UFGS-02 55 00, 16 

pp. http://www.wbdg.org/ccb/DOD/UFGS/UFGS% 

2002%2055%2000.pdf 

U.S. EPA. 1993a. Engineering Bulletin: Solidification/Stabilization of Organics and Inorganics, 

EPA 540/S-92/015. Office of Emergency and Remedial Response, 13 pp. http://www.cement.org/waste 

/wt_tech_epapubs.asp 

U.S. EPA. 1993b. Technical Resource Document: 

Solidification/Stabilization and Its Application to 

Waste Materials, EPA 530/R-93/012. Office of 

Research and Development, 372 pp. http://www. 

cement.org/waste/wt_tech_epapubs.asp 

U.S. EPA. 1996. Technology Screening Guide for 

Radioactively Contaminated Sites, EPA 402/R-96 

/017. Office of Radiation and Office of Solid Waste 

and Emergency Response, 203 pp. http://www.epa. 

gov/superfund/resources/radiation/pdf/techguide.pdf 

U.S. EPA. 1997. Innovative Site Remediation 

Design and Application, Volume 4: Stabilization/ 

Solidification, EPA 542/B-97/007. Office of Solid 

Waste and Emergency Response, 234 pp. 

http://nepis.epa.gov/pubtitleOSWER.htm 

U.S. EPA. 1998. MOLECULAR BONDING 

SYSTEM® Innovative Technology Evaluation Report, EPA 540/R-97/507. Office of Research and 

Development, 56 pp. http://www.epa.gov/ORD/ 

SITE/reports/540r97507/540r97507.pdf 

8 
U.S. EPA. 1999. Solidification/Stabilization Resource Guide, EPA 542/B-99/002. Office of Solid 

Waste and Emergency Response, 91 pp. http://www. 

clu-in.org/download/remed/solidstab.pdf 

U.S. EPA. 2000. Solidification/Stabilization Use at 

Superfund Sites, EPA 542/R-00/010. Office of Solid 

Waste and Emergency Response, 24 pp. http://www. 

clu-in.org/s.focus/c/pub/i/611/ 

U.S. EPA. 2002. Superfund Innovative Technology 

Evaluation Program: Evaluation of Soil Amendment 

Technologies at the Crooksville/Roseville Pottery 

Area of Concern, Rocky Mountain Remediation Services EnvirobondTM Process. Office of Research and 

Development, 16 pp. http://www.epa.gov/ORD/ 

NRMRL/pubs/540r02501/540R02501.pdf 

U.S. EPA. 2003. Evaluation of Soil Amendment 

Technologies at the Crooksville/Roseville Pottery 

Area of Concern STAR Organics Soil Rescue: 

Innovative Technology Evaluation Report, EPA 

540/R-99/501. Office of Research and Development, 

14 pp. http://www.epa.gov/ORD/SITE/reports/540 

r99501/540R99501prel.pdf 

U.S. EPA. 2004. Innovative Technology Evaluation 

Report: Stabilization of Mercury in Waste Material 

from the Sulfur Bank Mercury Mine. Office of 

Research and Development, 68 pp. http://www.epa. 

gov/ORD/NRMRL/pubs/540r04502/540r04502.pdf 

3.1.3 Chemical Oxidation 

Chemical oxidation typically involves reduction/ 

oxidation (redox) reactions that chemically convert 

hazardous contaminants to nonhazardous or less toxic 

compounds that are more stable, less mobile, or inert. 

Redox reactions involve the transfer of electrons from 

one chemical to another. Specifically, one reactant is 

oxidized (loses electrons) and one is reduced (gains 

electrons). There are several oxidants capable of 

degrading contaminants. Commonly used oxidants 

include potassium or sodium permanganate, Fenton’s 

catalyzed hydrogen peroxide, hydrogen peroxide, 

ozone, and sodium persulfate. Each oxidant has 

advantages and limitations, and while applicable to 

soil contamination and some source zone 

contamination, they have been applied primarily 

toward remediating groundwater. Several key 

concepts in oxidant selection for site cleanup include: 

• Is the oxidant capable of degrading the contaminant of concern? Is a catalyst or other additive 

required to increase effectiveness? 

• What is the soil oxidant demand (SOD)? SOD is 

a measure of how the naturally occurring materials in soil will affect the performance of 

some of the oxidants. For non-selective oxidants, high SOD will increase the cost of cleanup, as more oxidant will be required. 

• What is the naturally occurring pH of the soil/ 

groundwater system? Some oxidants require an 

acidic environment to work. If the soil is basic, 

an acid needs to be applied in addition to the 

oxidant. 

• How will the decomposition rate of the oxidant 

affect application strategies? Some unreacted 

oxidants may remain in the subsurface for 

weeks to months, while others naturally decompose within hours of injection. 

The type of delivery system selected depends upon 

the depth of the contaminants, the physical state of 

the oxidant (gas, liquid, solid), and its decomposition rate. Backhoes, trenchers, and augers have 

been used to work liquid and solid oxidants into 

contaminated soil and sludge. Liquids can be 

delivered either by gravity through wells and 

trenches or by injection. For vadose zones, gravity 

has the drawback of a relatively small area of 

influence. Pressurized injection of liquids or gases, 

either through the screen of a well or the probe of a 

direct push (DP) rig, will force the oxidant into the 

formation. The DP rig offers a cost-effective way of 

delivering the oxidant, and if needed, the hole can be 

completed as a small diameter well for later 

injections. Potassium permanganate and other solid 

phase chemical oxidants have also been added by 

hydraulic or pneumatic fracturing. 

The site stratigraphy plays an important role in the 

distribution of oxidants. Fine-grained units redirect 

oxidants to more permeable areas and are difficult to 

penetrate; hence, they can be the source of rebound 

later on, as contaminants diffuse out. Long-lived 

oxidants (e.g., permanganate) have the potential to 

remain active as this diffusion occurs, and they can 

mitigate some of the potential rebound. 

9 
Chemical oxidation usually requires multiple applications. Table 2 provides a qualitative list of oxidant 

reactivities with contaminants commonly found at 

sites. 

In the special case of nonaqueous phase liquids, 

oxidants that are in a water-based solution will only 

be able to react with the dissolved phase of the contaminant, since the two will not mix. This property 

limits their activity to the oxidant solution/NAPL 

interface. 

Cost estimates depend on the heterogeneity of the site 

subsurface, soil oxidation demand, stability of the 

oxidant, and type and concentration of the 

contaminant. Care should be taken when comparing 

different technologies on a cubic yard basis without 

considering these site attributes. Cost data can be 

found in ITRC (2005) and Brown (2003). In situ 

chemical oxidation has been used at a number of sites 

and is available from a variety of vendors. 

Sodium or Potassium Permanganate. Permanganate 

is a non-specific oxidizer of contaminants with low 

standard oxidation potential and high SOD. It can be 

used over a wide range of pH values and does not 

require a catalyst. Permanganate tends to remain in 

the subsurface for a long time, allowing for more 

contaminant contact and the potential of reducing 

rebound. As permanganate oxidizes organic 

materials, manganese oxide (MnO2) forms as a dark 

brown to black precipitate. During the treatment of 

large bodies of NAPL with high concentrations of 

permanganate, this precipitate may form a coating 

that reduces contact between oxidant and NAPL. 

The extent to which this reduction negatively 

impacts contaminant oxidation has not been 

quantified. Potassium permanganate has a much 

lower solubility than sodium and is generally 

applied at lower concentrations. Commercial-grade 

permanganates may contain elevated concentrations 

of heavy metals, and they may lower the pH of the 

treated zone (U.S. EPA 2004). If bioremediation is 

planned as a polishing step, permanganate will have 

an adverse effect on microbial activity and may 

cause a change in microbe distribution. This effect 

is generally transitory. Also, there is some evidence 

that permanganates may be inhibitory to Dehalococcoides ethenogenes, the microbial species that 

completely dechlorinates tetrachloroethene (PCE) 

and trichloroethene (TCE) (Hrapovic et al. 2005). 

Fenton’s Catalyzed Hydrogen Peroxide. Fenton’s 

reagent uses hydrogen peroxide in the presence of 

ferrous sulfate to generate hydroxyl radicals that are 

powerful oxidants. The reaction is fast, releases 

oxygen and heat, and can be difficult to control. 

Because of the fast reaction, the area of influence 

around the injection point is small. In conventional 

Table 2. Reactivity of Oxidants with Commonly Found Contaminants1 

Oxidant High Moderate Low 

Ozone PCE, TCE, DCE, VC, MTBE, 

CB, PAHs, Phenols, Explosives, 

PCBs, Pesticides 

BTEX, CH2Cl2, CT, CHCl3, 

Hydrogen Peroxide2 PCE, TCE, DCE, VC, CB, 

BTEX, MTBE, Phenols 

DCA, CH2Cl2, PAHs, 

Explosives 

TCA, CT, CHCl3, 

PCBs, Pesticides 

Calcium Peroxide PCE, TCE, DCE, VC, CB DCA, CH2Cl2 CT, CHCl3 

Fenton’s Reagent PCE, TCE, DCE, VC, CB, 

BTEX, MTBE, Phenols 

DCA, CH2Cl2, PAHs, 

Explosives 

TCA, CT, CHCl3, 

PCBs, Pesticides 

Potassium/Sodium 

Permanganate 

PCE, TCE, DCE, VC, TEX, 

PAHs, Phenols, Explosives 

Pesticides Benzene, DCA, 

CH2Cl2, TCA, CT, CB, 

CHCl3, PCBs 

Sodium Persulfate 

(Iron) 

PCE, TCE, DCE, VC, CB, 

BTEX, Phenols 

DCA, CH2Cl2, CHCl3, 

PAHs, Explosives, 

Pesticides 

TCA, CT, PCBs 

Sodium Persulfate 

(Heat) 

All CVOCs, BTEX, MTBE, 

PAHs, Phenols, Explosives, 

PCBs, Pesticides 

Source: ITRC 2005 and Brown 2003 

1

 Contaminant names are spelled out in the abbreviations and acronyms list in Section 5.0. 2 Peroxide without a catalyst must be applied at higher concentrations, which are inherently hazardous, and the reactions are 

10 
application, the reaction needs to take place in an 

acidified environment, which generally requires the 

injection of an acid to lower the treatment zone pH to 

between three and five. The reaction oxidizes the 

ferrous iron to ferric iron and causes it to precipitate, 

which can result in a loss of permeability in the soil 

near the injection point. Over time, the depletion of 

the ferrous ion can be rate limiting for the process. 

Chelated iron can be used to preserve the iron in its 

ferrous state at neutral pH, thus eliminating the acid 

requirement. The byproducts of the reaction are relatively benign, and the heat of the reaction may cause 

favorable desorption or dissolution of contaminants 

and their subsequent destruction. It also may cause 

the movement of contaminants away from the treatment zone or allow them to escape to the atmosphere. There are safety concerns with handling 

Fenton’s reagent on the surface, and the potential 

exists for violent reactions in the subsurface. In many 

cases there may be sufficient iron or other transition 

metals in the subsurface to eliminate the need to add 

ferrous sulfate. 

Hydrogen Peroxide. While catalysts can be added to 

increase oxidation potential, hydrogen peroxide can 

be used alone to oxidize contaminants. Peroxide 

oxidation is an exothermic reaction that can generate 

sufficient heat to boil water. The generation of heat 

can assist in making contaminants more available for 

degradation as well as allowing them to escape to the 

surface. With its high reaction and decomposition 

rates, hydrogen peroxide is not likely to address 

contaminants found in low permeability soil. Solid 

peroxides (e.g., calcium peroxide) in slurry form 

moderate the rate of dissolution and peroxide 

generation, thereby allowing a more uniform 

distribution. 

Ozone. Ozone, which is one of the stronger oxidants, 

can be applied as a gas or dissolved in water. As a 

gas, ozone can directly degrade a number of 

chemicals in both the dissolved and pure forms, and 

it provides an oxygen-rich environment for 

contaminants that degrade under aerobic conditions. 

It also degrades in water to form radical species, 

which are highly reactive and non-specific. Ozone 

may require longer injection times than other 

oxidants, and vapor control equipment may be needed 

at the surface. Because of its reactivity, ozone may 

not be appropriate for slow diffusion into lowpermeability soil. 

Sodium Persulfate. Persulfate (S2O8

-2) is a strong 

oxidant with a higher oxidation potential than 

hydrogen peroxide and a potentially lower SOD than 

permanganate or peroxide. Persulfate reaction is 

slow unless placed in the presence of a catalyst, such 

as ferrous iron, or heated to produce sulfate free 

radicals (•SO4

-

) that are highly reactive and capable 

of degrading many organic compounds. At 

temperatures above 40o

C, persulfate becomes 

especially reactive and can degrade most organics 

(Block et al. 2004). Like Fenton’s reagent, the 

ferrous iron catalyst (when used) will degrade with 

time and precipitate. 

Cited and Other References 

Block, P., R. Brown, and D. Robinson. 2004. Novel 

activation technologies for sodium persulfate in situ 

chemical oxidation. Proceedings of the Fourth International Conference on the Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA. 

http://www.geo-log.de/uploads/media/novel_ 

persulfate_activation_technologies.pdf 

Brown, R. 2003. In situ chemical oxidation: 

performance, practice, and pitfalls. 2003 AFCEE 

Technology Transfer Workshop, February 25, 2003, 

San Antonio, TX. http://www.cluin.org/download/ 

techfocus/chemox/4_brown.pdf 

Haselow, J., P. Block, and F. Sessa. 2006. Pilot scale 

application of heat-activated persulfate at a former 

petroleum underground storage tank area. The 22nd 

International Conference on Soils, Sediments, and 

Water, University of Massachusetts, Amherst, MA, 

October 16-19, 2006. 

Hrapovic, L. et al. 2005. Laboratory study of treatment of trichloroethene by chemical oxidation followed by bioremediation. Environ. Sci. Technol. 

39(8): 2888-2897. 

Huling, S. and B. Pivetz. 2006. Engineering Issue: 

In Situ Chemical Oxidation, EPA 600/R-06/072. 

U.S. EPA, Office of Research and Development, 60 

pp. http://www.epa.gov/ada/topics/oxidation_issue. 

html 

ITRC. 2005. Technical and Regulatory Guidance for 

In Situ Chemical Oxidation of Contaminated Soil 

and Groundwater, 2nd Edition.172 p. http://www.itrc 

web.org/Documents/ISCO-2.pdf 

11 
NAVFAC. October 2006. In Situ Chemical Oxidation 

Multi-media Training Tool. http://www.ert2.org/ert2 

portal/DesktopDefault.aspx 

U.S. EPA. 2004. How to Evaluate Alternative 

Cleanup Technologies for Underground Storage Tank 

Sites: A Guide for Corrective Action Plan Reviewers, 

EPA 510/R-04/002. Office of Underground Storage 

Tanks. http://epa.gov/oust/pubs/tum_ch13.pdf 

3.1.4 Soil Flushing 

Soil flushing involves flooding a zone of contamination with an appropriate solution to remove the 

contaminant from the soil. Water or liquid solution is 

injected or infiltrated into the area of contamination. 

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 and brought to the surface for disposal, 

recirculation, or on-site treatment and reinjection. 

Application of soil flushing relies on the ability to 

deliver, control the flow, and recover the flushing 

fluid. 

Flushing solutions may be water, acidic aqueous 

solutions, basic solutions, chelating or complexing 

agents, reducing agents, cosolvents, or surfactants. 

Water will extract water-soluble (hydrophilic) or 

water-mobile constituents. Acidic solutions may 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 used to 

recover some metals. Cosolvents are usually miscible 

and are effective for some organics. Surfactants can 

assist in the removal of hydrophobic organics (U.S. 

EPA 1991). 

The techniques employed the most in soil flushing are 

surfactant and cosolvent flooding for fuels and 

chlorinated solvents. There are many types of 

surfactants (cationic, anionic, nonionic), and while 

adjustments can be made in the fluid composition, 

anionic or nonionic surfactants are generally used. 

This is because their negative or neutral charge 

reduces the possibility of their sorption to negatively 

charged clay particles. They also are generally less 

toxic than cationic surfactants. 

Surfactants are commonly constructed with hydrophobic and hydrophilic chemical components, 

meaning that one end of the molecule is attracted to 

oil (or organic compounds) and the other to water. 

Surfactants chosen primarily to increase the 

contaminant (generally a NAPL) solubility are used 

in a solubilization flood. Surfactants chosen to 

produce ultra-low interfacial tensions are employed 

in a mobilization flood (Kueper et al. 1997). Mobilization flooding should only be considered when 

there is a high degree of certainty that the solution 

can be recovered, such as with a competent bedrock 

or capillary barrier underlying the treatment zone. 

A typical surfactant solution also may contain 

additives, such as electrolytes and a cosolvent. In 

addition to being effective with the target contaminant, the surfactant solution also should be compatible with the site-specific soil, soil pore water, 

and groundwater (if applicable). A cosolvent, such 

as isopropanol, can be used to improve the surfactant 

solubility in solution and provide the surfactant/ 

contaminant solution with an acceptable viscosity. A 

side effect of adding chemicals to the surfactant 

solution is that they need to be treated along with the 

contaminant at the recovery end (NAVFAC 2002). 

Cosolvents, usually alcohols, are chemicals that 

dissolve in both water and NAPL. In an alcohol 

flood, the alcohol may partition into both the NAPL 

and water phases. Partitioning affects the viscosity, 

density, solubility, and interfacial tension of the 

NAPL (Kueper et al. 1997). The physical properties 

of the NAPL vary with the amount of alcohol available for interaction, and whether the alcohol 

preferentially dissolves into the NAPL or into the 

water. Complete miscibility is achievable and results 

in a pumpable solution that, depending upon the 

density of the NAPL and the proportions of alcohol 

and water in the solution, may be more or less dense 

than water. 

Before implementing surfactant and/or cosolvent 

flushing, laboratory and bench-scale treatability 

testing should be done to ensure the selection of an 

agent(s) best suited for the contaminant and the sitespecific soil and geochemical conditions. Modeling 

of subsurface conditions is commonly done to 

ensure the best delivery system. Flushing is most 

efficient in relatively homogeneous and permeable 

(K $ 10-3 cm/sec) soil (NAVFAC 2002). Heterogeneous soil reduces the efficiency of the flood sweep 

and may prevent optimum contact between the 

12 
agent(s) and the target contaminant. Flushing of 

relatively homogeneous but lower permeability (10-4 

to 10-5 cm/sec) units is possible, but it requires a highinduced gradient to move the agent, while greatly 

increasing the remediation time (NAVFAC 2002). 

Other soil factors that may adversely affect efficiency 

are high cation exchange capacity, high buffering 

capacity, high organic soil content, and pH. 

Land disposal restrictions and underground injection 

control regulations also may limit selection of the 

flushing solution. At a former drycleaner, ethanol was 

substituted for isopropanol because of regulatory 

concern about the toxicity and persistence of 

isopropanol. Most states allow in situ flushing of 

saturated or unsaturated soil, with a permit, if the 

aquifer in the area is already contaminated. When 

applying for a permit, all chemicals involved, including unreacted compounds and impurities, must be 

listed (NAVFAC 2002). 

An example of an alcohol flood to address PCE contamination was carried out at the former Sage’s Dry 

Cleaners in Jacksonville, FL. The depth to groundwater at the site was eight feet with the treatment 

zone consisting of a 24-ft by 9-ft elliptical source area 

at 26 to 31 ft below ground surface (bgs). About 

9,000 gallons of a 95 percent ethanol/5 percent water 

solution were injected into the target zone. 

Approximately 160,000 gallons of a ternary mixture 

of PCE/ethanol/water were treated on site to remove 

the PCE. The ethanol/water solution was disposed of 

offsite. Forty-two liters of PCE were recovered, 

which represented approximately 63 percent of the 

estimated volume (Florida Department of 

Environmental Protection 1998). Ethanol had an 

advantage in that it could be left in the ground at 

elevated levels while other alcohols, such as 

isopropanol, would have had to be contained due to 

their toxicity. The residual ethanol formed an organic 

substrate that promoted subsequent microbial 

reductive dechlorination of the remaining PCE. The 

authors of the study noted that overall cost could have 

been lowered had they recovered the ethanol and 

recycled it (Florida Department of Environmental 

Protection 1998). 

Due to its use in oil field applications, soil flushing is 

considered a mature technology; however, it has 

found limited application in the environmental arena. 

ITRC (2003) estimates the cost of surfactant/cosolvent flushing of a DNAPL source zone to range 

between $65 and $200 per cubic yard. Cost estimates 

of $100 to $300 per cubic yard for flushing are 

given on the NAVFAC website. The variability 

stems from the waste type and the quantity to be 

treated. The NAVFAC figures do not include design 

and engineering costs, which can be considerable. 

Cost per cubic yard can be misleading, and the cost 

per gallon recovered or destroyed should also be 

evaluated. 

Cited and Other References 

Florida Department of Environmental Protection. 

1998. Cosolvent Flushing Pilot Test Report, Former 

Sage’s Dry Cleaner, 58 pp. http://clu-in.org/down 

load/remed/sages.pdf 

ITRC. 2003. Technical and Regulatory Guidance for 

Surfactant/Cosolvent Flushing of DNAPL Source 

Zones. 140 pp. http://www.itrcweb.org/gd_DNA 

PLs.asp 

Kueper, B. et al. 1997. Technology Practices Manual for Surfactants and Cosolvents (TR-97-2). Advanced Applied Technology Demonstration Facility 

Program, Rice University. http://www.cluin.org/ 

PRODUCTS/AATDF/Toc.htm 

NAVFAC. 2006. Naval Engineering Facilities 

Environmental Restoration & BRAC website. 

https://portal.navfac.navy.mil/portal/page?_pageid= 

181,5346904&_dad=portal&_schema=PORTAL 

NAVFAC. 2002. Surfactant-Enhanced Aquifer Remediation (SEAR) Design Manual, NFESC Technical Report TR-2206-ENV, 110 pp. http://enviro. 

nfesc.navy.mil/erb/erb_a/restoration/technologies/ 

remed/phys_chem/sear/tr-2206-sear.pdf 

NAVFAC. 2003. Surfactant-Enhanced Aquifer Remediation (SEAR) Implementation Manual, NFESC 

Technical Report TR-2219-ENV, 54 pp. http:// 

enviro.nfesc.navy.mil/erb/erb_a/restoration/ 

technologies/remed/phys_chem/sear/tr-2219­

sear.pdf 

U.S. EPA. 1991. Engineering Bulletin: In Situ Soil 

Flushing, EPA 540/2-91/021. Office of Research 

and Development, 7 pp. http://nepis.epa.gov/pubtitle 

OSWER.htm 

U.S. EPA. 1996. Environmental Research Brief: 

Surfactant-Enhanced DNAPL Remediation: Sur13 
factant Selection, Hydraulic Efficiency, and Economic Factors, EPA 600/S-96/002. Office of Research 

and Development, 15 pp. http://nepis.epa.gov/pub 

titleORD.htm 

3.1.5 Electrokinetic Separation 

Electrokinetic separation is an emerging technology 

that relies on the application of a low-intensity, direct 

current through the soil to separate and extract heavy 

metals, radionuclides, and organic contaminants from 

unsaturated soil, sludge, and sediment. The current is 

applied across electrode pairs that have been implanted in the ground on each side of the contaminated 

soil mass. During electromigration, positively 

charged chemical species, such as metals, ammonium 

ions, and some organic compounds, move toward the 

cathode, and negatively charged chemicals, such as 

chloride, cyanide, fluoride, nitrate, and negativelycharged organic species, migrate toward the anode 

(Figure 3). Electromigration does not require 

advective flow of pore water for the chemical species 

to move. In fine-grained soil, the electric current also 

causes electroosmosis, which is an electrically 

induced hydraulic flow of ground or soil pore water 

between the electrodes. This flow can carry neutrally 

charged species with it. Suspended, charged colloids 

and miscelles can also move by electrokinetics 

through the process of electrophoresis. Electrophoresis, in this instance, is similar to electromigration except that the species moving are not 

single molecules. 

Electrolysis reactions (conversion of electrical 

energy into chemical potential energy) create H2 and 

OHat the cathode and O2 and H+

 at the anode. These 

reactions create an acid front near the anode and a 

base front near the cathode that migrate towards each 

other. The acid front aids in increasing the mobility 

of cationic species, but in some soils, it can retard 

electroosmois (Saichek and Reddy 2005). The hydroxide front needs to be controlled to avoid the 

premature precipitation of some target metal ions. 

This technology can be applied to contaminant concentration ranges from a few ppm to greater than 

10,000 ppm, but may not be effective for treating 

multiple contaminants that have significantly different concentrations. The target compounds are either 

extracted to a recovery system or deposited at the 

electrode. Surfactants and complexing agents may be 

used to increase solubility and assist in the movement 

of the contaminant, although care should be taken 

when choosing between charged (anionic/cationic) 

and neutral surfactants. When electroosmotic flow is 

from the anode to the cathode, the flow will assist 

cationic species and retard anionic ones (Saichek 

and Reddy 2005). 

For the electrokinetics to work, the soil moisture 

must be conductive and sufficient to allow electromigration but, optimally, not saturated. Removal 

efficiencies are directly related to the solubility of 

the target contaminant (which can be amended with 

surfactants), its electrical charge, and its concentration relative to other ions or contaminant species 

(Van Cauwenberghe 1997). 

Unfavorable conditions at a site include soil with a 

high cation exchange capacity, high buffering capacity, high naturally occurring organic content, 

salinity, and very low moisture content. The presence of subsurface metal structures or utilities can 

also adversely affect performance. 

Figure 3. Simple Electrokinetic Separation 

System 

Electrokinetic separation has been demonstrated at 

several sites with mixed results. An independent 

evaluation was performed at the Department of 

Energy (DOE) Sandia National Laboratories in 

Albuquerque, New Mexico for their patented process with Cr(VI) as the target contaminant (U.S. 

EPA 1999). Also, a field test was conducted by the 

Navy at Point Mugu with one conclusion being that 

there was a large discrepancy between what was 

expected from the bench study, which showed the 

technology would be very effective, versus what was 

actually obtained in the field, where the technology 

performed poorly (ESTCP 2000). 

14 
A system that uses in situ treatment combined with 

electrokinetic separation is the LasagnaTM technique. 

In this system electrode arrays and treatment zones 

(e.g., crushed limestone, zero valent iron) are 

interlayered. The applied current causes the contaminants to move through the treatment zones where 

they are either destroyed or immobilized. LasagnaTM 

was applied with some success to treat a TCE 

contaminated clay soil at the DOE gaseous diffusion 

plant in Paducah, Kentucky (U.S. DOE 2002). 

Because of the limited application of electrokinetic 

separation, reliable cost data for full-scale applications are scarce. Costs will vary significantly depending upon the concentration of the target contaminant, presence of non-target ions, and soil characteristics and moisture content. Estimates from three 

vendors were collected by Van Cauwenberghe (1997) 

and ranged from $20 to $100 per cubic yard for one 

vendor to $60 to $225 per cubic yard for the high 

vendor estimate. 

Cited and Other References 

Alshawabkeh, A. 2001. Basics and Applications of 

Electrokinetic Remediation. Northeastern University, 95 pp. http://www1.coe.neu.edu/~aalsha/short 

course.pdf 

Environmental Security Technology Certification 

Program (ESTCP). 2000. Final In-Situ Electrokinetic 

Remediation of Metal Contaminated Soils 

Technology Status Report, SFIM-AEC-ET-CR99022. US Army Environmental Center, 30 pp, July 

2000. http://www.estcp.org/documents/techdocs/ 

ISERMCS_Report.pdf 

NAVFAC. 2000. TechData Sheet: A Demonstration 

of Electrokinetic Remediation, TDS-2084-ENV, 2 pp. 

Roulier; M., M. Kemper; P. Cluxton. 2002. 

Horizontal Configuration of the LasagnaTM Treatment Technology. User Guide, EPA 600/R-02/033. 

U.S. Environmental Protection Agency, Office of 

Research and Development, 38 pp. http://nepis.epa. 

gov/pubtitleORD.htm 

Saichek, R. and K. Reddy. 2005. Electrokinetically 

enhanced remediation of hydrophobic organic compounds in soils: A review. Critical Reviews in Environmental Science and Technology, 35: 115–192. 

http://www.uic.edu/classes/cemm/cemmlab/35-2­

2005.pdf 

U.S. DOE. 2002. Final Remedial Action Report for 

LasagnaTM Phase IIb In-Situ Remediation of Solid 

Waste Management Unit 91 at the Paducah Gaseous 

Diffusion Plant, Paducah, Kentucky, OR/072037 

&D1, 80 pp. http://www.rtdf.org/public/lasagna/ 

lasagna_final_a.pdf 

U.S. EPA. 1997. Electrokinetic Laboratory and Field 

Processes Applicable to Radioactive and Hazardous 

Mixed Waste in Soil and Groundwater. EPA 402/R97/006. Office of Radiation and Indoor Air. July 

1997. http://nepis.epa.gov/pubtitleOAR.htm 

U.S. EPA. 1999. Sandia National Laboratories In 

Situ Electrokinetic Extraction Technology Innovative Technology Evaluation Report, EPA 540/R97/509. Office of Research and Development, 69 pp. 

http://www.epa.gov/ORD/SITE/reports/540r97509/ 

540r97509.pdf 

Van Cauwenberghe, L. 1997. Electrokinetics. Technology Overview Report. Ground Water Remediation Technologies Analysis Center. 

3.2 Biological Treatment Technologies 

Biological treatment involves the use of microorganisms or vegetation (phytoremediation). Many 

naturally occurring microorganisms (typically, 

bacteria and fungi) can transform hazardous chemicals to substances that may be less hazardous than 

the original compounds. Microrganisms also have 

been used to alter the valence of some hazardous 

metals (e.g., Cr(VI)), thereby making them less 

hazardous and less mobile. Several plant species 

have the ability to bioaccumulate heavy metals 

found in the soil, and some tree species can 

sequester, destroy, and/or evapotranspire various 

organic compounds. 

Microbial bioremediation occurs under both aerobic 

and anaerobic conditions and at contaminated sites 

as either intrinsic and/or enhanced biodegradation. 

Intrinsic bioremediation depends on indigenous 

microorganisms to degrade contaminants without 

any amendments. Monitored natural attenuation 

(MNA) often relies on intrinsic bioremediation as an 

important removal mechanism. During enhanced 

bioremediation, biodegradation is facilitated by 

manipulating the microbial environment. Typically, 

the environment is manipulated by supplying 

amendments, such as air, organic substrates, nutrients, and other compounds, whose absence limit 

15 
treatment. In some cases, bioremediation has been 

enhanced by adding microbial cultures (bioaugmentation). 

3.2.1 Bioventing 

Bioventing involves the injection of a gas into the 

subsurface to enhance the biodegradation of a 

contaminant. The gas can be used to keep the subsurface aerobic or anaerobic, or to provide a substrate that enables cometabolic degradation to occur. 

Aerobic Bioventing 

Aerobic bioventing has a robust track record in 

treating aerobically degradable contaminants, such as 

fuels. Bioventing involves supplying oxygen to 

contaminated unsaturated soils with low oxygen 

concentrations to facilitate aerobic microbial biodegradation. Using the supplied oxygen, the microbes 

oxidize the contaminants to gain energy and carbon 

for growth. Oxygen is typically introduced by air 

injection wells that push air into the subsurface. 

Aerobically degradable contaminants may be treated 

by bioventing, but fuels have received the most 

attention. The U.S. Air Force Bioventing Initiative 

and the U.S. EPA Bioremediation Field Initiative 

evaluated bioventing at 125 sites contaminated by 

petroleum hydrocarbons. At sites where initial studies 

were positive, pilot-scale bioventing was installed 

and operated for one year. The experience from 

bioventing demonstrations at these sites was 

condensed into a manual (U.S. EPA 1995a & 1995b). 

The manual contains information on bioventing 

principles; site characterization; field treatability 

testing; system design, operation, and installation; site 

closure; and techniques to demonstrate the extent and 

mechanism for contaminant removal. Based on this 

research, bioventing proved to be an economical and 

effective method to treat unsaturated soil 

contaminated by petroleum products. Regulatory 

acceptance of this technology has occurred in 30 

states and in all 10 EPA regions. The use of this 

technology in the private sector has increased 

following the U.S. Air Force Bioventing Initiative 

and the U.S. EPA Bioremediation Field Initiative. 

Estimated costs range from $10 to $60 per cubic yard 

(U.S. EPA 1995b). 

In addition to fuels, aerobic bioventing has treated a 

variety of other contaminants, including nonhalogenated solvents, such as benzene, acetone, toluene, 

and phenol; lightly halogenated solvents, such as 

1,2-dichloroethane, dichloromethane, and chlorobenzene; and SVOCs, such as some PAHs (Figure 

4). The principles outlined in the manual are also 

applicable for aerobically degradable non-fuel 

contaminants, but since the experience with these 

other types of contaminants is more limited, more 

information may be needed. For example, laboratory 

and pilot-scale studies may be needed to evaluate 

effectiveness, design the bioventing system, estimate treatment times, and demonstrate that biodegradation is the primary mechanism of removal. In 

evaluating the feasibility of treating other 

contaminants, the key is to understand the volatility 

relative to the biodegradability. 

Bioventing is typically operated in air injection 

mode to alleviate low oxygen levels in the 

subsurface. The injection system should be designed 

considering soil gas permeability, contaminant 

diffusion and distribution, and environmental 

factors, such as moisture content, pH, temperature, 

and electron acceptor conditions. When building 

foundations or similar structures are close to the site, 

vacuum extraction wells, which draw air through the 

subsurface, may be used to avoid the buildup of 

contaminated, and possibly explosive, vapors in the 

building basements. 

Extracted gases require treatment since volatile compounds may be removed from the ground. In cases 

of remote locations without electric power, passive 

air delivery systems may be used. These systems use 

one-way valves and changes in barometric pressure 

to deliver air to the subsurface; however, passive 

systems may have longer treatment times depending 

on the quantities of air supplied to the subsurface. 

Compared to soil vapor extraction, all bioventing 

delivery systems employ lower air flow rates that 

provide only the amount of oxygen required to 

enhance removal. When operated properly, the low 

flow rates of air injection do not result in the release 

of the contaminants into the atmosphere through 

volatilization. 

To determine if bioventing is appropriate at a 

specific site, existing site data should be evaluated 

and, if needed, additional data collected. For 

example, information about the types, quantities, and 

three-dimensional distribution of contaminants is 

needed. This includes the presence and location of 

free product and whether there is a chance of 

16 
continuing contamination from leaking pipes or 

tanks. Bioventing alone is not sufficient to remediate 

sites with large quantities of free product or ongoing 

releases. Information about the historical water table 

levels and soil characteristics, such as gas 

permeability, is also needed. A soil gas survey can 

provide useful information, especially at sites with 

relatively shallow contamination (depths typically less 

than 20 ft). The soil gas 

survey is useful to determine 

whether oxygen-limited 

conditions exist. Low oxygen 

levels (less than five percent) 

are a good indicator that 

existing bacteria are capable 

of degrading the contaminants of concern, because 

soil gas in uncontaminated 

soil generally exhibits oxygen 

concentrations similar to 

ambient air. In addition, the 

soil gas survey can be useful 

in delineating the extent of 

contamination and identifying 

locations for vent wells and 

monitoring points. If this 

preliminary information looks 

promising, more specific information should be gathered, 

such as soil contaminant concentrations and distribution 

and soil characterization. 

Respiration rate, soil gas 

permeability, and oxygen 

radius of influence will be 

needed to properly design the 

system. 

Performance monitoring, after 

and amount of contaminant removed, oxygen 

supply, and carbon dioxide generation, as well as 

mass balances relating the three amounts, may be 

useful in establishing bioremediation as the primary 

mechanism of removal. For sites where non-fuel 

contaminants are to be treated by bioventing, other 

factors may be considered in establishing biological 

a bioventing system has been Figure 4. Amenability of Common Contaminants to Bioventing Technologies 

installed, typically includes (aerobic, anaerobic, and cometabolic) 

soil gas monitoring to ensure 

that the site is well oxygenated, in situ respiration 

testing to monitor the progress of remediation, and 

operation and maintenance of the bioventing system. 

At some sites, surface emissions sampling may be 

needed. At sites using extractive bioventing, the 

degree of volatilization versus biodegradation may be 

determined by measuring offgas concentrations. 

Injection-based systems may be briefly reconfigured 

to gain similar information. Measurements of the rate 

activity as the primary mechanism of removal. 

Finally, measurement of stable isotope ratios may be 

useful in qualitatively validating biodegradation as 

the mechanism of contaminant removal. This 

measurement is not required, but it is available to 

resolve regulatory concerns. 

Aerobic bioventing has proven to be a useful 

cleanup technology at many sites under a variety of 

17 
conditions, but like all technologies, bioventing has 

some limitations. One limitation revolves around the 

ability to deliver oxygen to the contaminated soil. For 

example, soil with an extremely high moisture 

content may be difficult to biovent due to reduced 

soil gas permeability. Similarly, low permeability 

soils limit the ability to distribute air through the 

subsurface; however, in both cases, the design of the 

bioventing system may compensate for low permeability. Sites with shallow contamination also pose a 

problem to bioventing because designing the system 

to minimize environmental release and achieve 

sufficient aeration, may be difficult. In this situation, 

operating in extraction mode may be needed. 

Another limitation is that aerobic bioventing will not 

stimulate contaminant removal if the contaminated 

zone is aerobic. If a soil gas survey measures soil 

oxygen levels consistently above five percent, then 

the soil is sufficiently aerated for biodegradation to 

occur, and oxygen is not limiting degradation. Bioventing will not enhance removal in this situation. 

This situation is unusual, and if encountered, may 

indicate that some other species, such as metals, is 

inhibiting degradation. 

While relatively inexpensive, aerobic bioventing can 

take a few years to clean up a site, depending on the 

contaminant concentrations and site-specific removal 

rates. For petroleum hydrocarbon sites, the heavier 

the product being treated, the longer the remediation 

time. If a quicker cleanup is needed, other technologies may be more appropriate. 

Anaerobic Bioventing 

While aerobic bioventing is useful for degrading 

many hydrocarbons, some chlorinated compounds are 

not effectively treated aerobically. Microbes may 

degrade these contaminants directly via anaerobic 

reductive dechlorination or through anaerobic 

cometabolic pathways. Anaerobic reductive dechlorination is a biological mechanism, typically marked by 

sequential removal of chlorine ions from a molecule. 

Microbes possessing this pathway gain energy from 

this process. In some situations, microorganisms 

fortuitously degrade contaminants, while gaining 

energy and carbon from other compounds 

(cometabolites). These organisms usually do not 

obtain any benefit from contaminant degradation, and 

the removal process is called cometabolism. Anaerobic bioventing may involve both anaerobic 

reductive dechlorination and anaerobic cometabolism to destroy the contaminants of concern. 

Anaerobic bioventing uses the same type of gas 

delivery system as aerobic bioventing, but instead of 

injecting air, nitrogen and electron donors (e.g., 

hydrogen and carbon dioxide) are used. The nitrogen 

displaces the soil oxygen, and the electron donor gas 

facilitates microbial dechlorination. Volatile and 

semivolatile organic compounds may be produced 

during anaerobic bioventing that are not 

anaerobically degradable. Volatile compounds may 

be aerobically degraded in the soil surrounding the 

treatment zone. Semivolatile compounds may be 

treated by following anaerobic bioventing with aerobic bioventing. Since aerobic and anaerobic bioventing share similar gas delivery systems, the switch 

can be made by simply changing the injected gas. 

Anaerobic bioventing is an emerging technology 

that may be useful in treating highly chlorinated 

compounds, such as PCE, TCE, pentachlorophenol 

(PCP), some polychlorinated biphenyls (PCBs), and 

pesticides, such as lindane and dichlorodiphenyltrichloroethane (DDT). Due to the limited experience with this technique, laboratory, pilot, and field 

demonstrations are recommended to apply this technology with confidence to remediate a site. 

Particular attention should be paid to the formation 

of degradation products and whether contaminants 

are converted to non-toxic compounds. For example, 

sites contaminated by PCE and TCE may not show 

complete dechlorination, rather dechlorination stalls 

at cis-1,2-dichloroethene (cis-DCE) or vinyl chloride 

(VC). Since VC is more toxic than the original 

contaminants, incomplete dechlorination would not 

be acceptable. The cis-DCE or VC stall may be due 

to the availability of an electron donor or the 

indigenous microbial community. If the electron 

donor is limited, additional donor should be added. 

If the indigenous culture is not able to completely 

dechlorinate the solvents, the site could be switched 

to another type of bioventing (e.g., cis-DCE is 

aerobically degradable through cometabolism, and 

VC is aerobically degradable). Laboratory testing 

can demonstrate whether complete dechlorination 

occurs at a site, provide information about suitable 

electron donors and the quantities required, estimate 

removal rates, and demonstrate whether hazardous 

byproducts are formed. 

18 
As with the other bioventing technologies, the ability 

to deliver gases to the subsurface is important. Soil 

with a high moisture content or low gas permeability 

may require careful design to deliver appropriate 

levels of nitrogen and electron donor. Sites with 

shallow contamination or nearby buildings are also a 

problem, since this technology is operated by 

injecting gases. In addition, anaerobic bioventing can 

take a few years to clean up a site depending on the 

contaminant concentrations and site-specific removal 

rates. If a quicker cleanup is needed, other technologies may be more appropriate. Finally, no 

rigorous cost models have been developed for 

anaerobic bioventing; however, the costs should be 

similar to aerobic bioventing with the following 

additional costs: laboratory treatability test and field 

testing; nitrogen and electron donor additions; and 

additional soil and gas analyses. 

Cometabolic Bioventing 

Cometabolic bioventing involves injecting air into the 

subsurface along with a suitable gaseous substrate to 

promote cometabolic reactions with the target 

compound. As with anaerobic cometabolism, some 

microorganisms fortuitously degrade contaminants 

while oxidizing other compounds (cometabolites) for 

energy and carbon. The organisms usually do not 

obtain any benefit from contaminant degradation. A 

suitable substrate should be determined in the 

laboratory but may include methane, ethane, propane, 

butane, and pentane. The delivery system is similar to 

other bioventing technologies and subject to many of 

the same limitations. Cometabolic bioventing is 

applicable to contaminants, such as TCE, trichloroethane (TCA), ethylene dibromide, and dichloroethene (DCE), that resist direct aerobic degradation. 

This technology is not applicable to PCE. 

The Bioremediation Consortium under the Remediation Technology Development Forum (RTDF) 

conducted cometabolic bioventing demonstrations at 

Dover and Hill Air Force Bases (AFB). At Dover 

AFB, a field demonstration of cometabolic bioventing was done at Building 719. The site was 

contaminated with fuel and solvents during engine 

inspection and maintenance operations. The targeted 

contaminants of the demonstration were TCE, as high 

as 250 mg/kg; TCA, 10 to 1,000 mg/kg; and DCE, 1 

to 20 mg/kg. Laboratory tests were used to select 

propane as the cometabolic substrate and predict that 

a substrate acclimation period would be needed. The 

test plot was acclimated to propane addition through 

pulsed propane/air injections for three months, and 

then the test plot was operated for 14 months with 

continuous propane injection. Concentrations of 

TCE, TCA, and DCE were reduced to less than 0.25, 

0.5 and 0.25 mg/kg, respectively. Soil chloride 

accumulation confirmed biodegradation as the 

mechanism of removal (U.S. EPA 2000). 

Because experience with cometabolic bioventing is 

limited, laboratory and pilot-scale studies are recommended to evaluate effectiveness, select a cometabolite, identify needs for acclimation periods, design 

the system, and estimate treatment times. Operational costs should be similar to those of aerobic 

bioventing except for the addition of the substrate 

gas and additional monitoring of soil and soil gas. 

Cited and Other References 

Abriola, L., J. Lang, and K. Rathfelder. 1997. 

Michigan Soil Vapor Extraction Remediation 

(MISER) Model: A Computer Program to Model 

Soil Vapor Extraction and Bioventing of Organic 

Chemicals in Unsaturated Geological Material, EPA 

600/R-97/099. U.S. EPA, Office of Research and 

Development, 260 pp. http://nepis.epa.gov/pubtitle 

ORD.htm 

Downey, D., R. Miller, and T. Dragoo. 2004. 

Procedures for Conducting Bioventing Pilot Tests 

and Long-Term Monitoring of Bioventing Systems. 

Air Force Center for Environmental Excellence. 

NTIS: ADA423587. 80 pp. http://stinet.dtic.mil/cgibin/GetTRDoc?AD=ADA423587&Location= 

U2&doc=GetTRDoc.pdf 

NAVFAC. 2003. FINAL REPORT: Addendum 

–Natural Pressure-Driven Passive Bioventing, 

TR-2221-ENV, 74 pp. http://enviro.nfesc.navy.mil/ 

erb/erb_a/restoration/technologies/remed/bio/tr-22 

21-biovent.pdf 

NAVFAC. 2000. Passive Bioventing in Stratified 

Soils and Shallow Groundwater Conditions, NFESC 

TDS-2083-ENV, 6 pp. http://enviro.nfesc.navy.mil/ 

erb/erb_a/restoration/technologies/remed/bio/tds2083biovent.pdf 

NAVFAC. 2003. Final Report: Addendum –Natural 

Pressure-Driven Passive Bioventing, TR-2221-ENV, 

74 pp. http://enviro.nfesc.navy.mil/erb/erb_a/restor 

ation/technologies/remed/bio/tr-2221-biovent.pdf 

19 
NAVFAC. 2006 (Nov). Bioventing Cost Estimator. 

Naval Facilities Engineering Command. Webpage. 

http://enviro.nfesc.navy.mil/erb/erb_a/restoration/ 

technologies/remed/bio/BVCE.XLS 

Shah, J.K., G.D. Sayles, M.T. Suidan, P. Mihopoulos, 

and S. Kaskassian. 2001. Anaerobic bioventing of 

unsaturated zone contaminated with DDT and DNT. 

Water Sci Technol. 2001;43(2):35-42. Abstract at 

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi? 

cmd=Retrieve&db=PubMed&list_uids=11380 

202&dopt=Abstract 

Shewfelt, K., H. Lee, and R. Zytner. 2005. 

Optimization of nitrogen for bioventing of gasoline 

contaminated soil. J. Environ. Eng. Sci. 4(1): 29-42. 

http://pubs.nrc-cnrc.gc.ca/rp/rppdf/s04-040.pdf 

U.S. EPA. 1995a. Manual: Bioventing Principles and 

Practices, Volume I: Bioventing Principles, EPA 540/ 

R-95/534a. Office of Solid Waste and Emergency 

Response and Office of Research and Development, 

94 pp. http://nepis.epa.gov/pubtitleOSWER.htm 

U.S. EPA. 1995b. Manual: Bioventing Principles and 

Practices, Volume II: Bioventing Design, EPA 

540/R-95/534b. Office of Solid Waste and Emergency Response and Office of Research and 

Development, 94 pp. http://nepis.epa.gov/pubtitle 

OSWER.htm 

U.S. EPA. 2000. Cost and Performance Report: 

Cometabolic Bioventing at Building 719, Dover Air 

Force Base Dover, Delaware. Office of Soild Waste 

and Emergency Response, 6 pp. http://costperform 

ance.org/pdf/Dover719Bio.PDF 

U.S. EPA. 2002. Innovative Technology Evaluation 

Report: Envirogen Propane Biostimulation Technology for In-Situ Treatment of MTBE Contaminated Ground Water, EPA 600/R02/092. Office of 

Research and Development, 152 pp. http://www. 

epa.gov/ada/download/reports/600R02092/600 

R02092-full.pdf 

U.S. EPA. 2006. In Situ and Ex Situ Biodegradation 

Technologies for Remediation of Contaminated Sites, 

EPA 600/R-06/015. http://www.epa.gov/nrmrl/pub 

lications.html 

3.2.2 Phytoremediation 

Phytoremediation uses plants to extract, degrade, 

contain, or immobilize contaminants in soil, groundwater, and other contaminated media. The phytoremediation mechanisms used to treat contaminated 

soil in situ are phytoextraction, rhizodegradation, 

phytodegradation, phytovolatilization, and phytostabilization. 

Phytoremediation is best used to treat large areas of 

shallow contamination. Because high levels of 

contaminants may be toxic to plants and inhibit their 

growth, phytoremediation is best applied to low and 

moderate levels of contamination, used in conjunction with other treatment methods, or used as a final 

polishing step in site remediation. 

The various mechanisms of phytoremediation can 

treat a wide range of contaminants, including metals, 

VOCs, PAHs, petroleum hydrocarbons, radionuclides, and munitions, although not all mechanisms are applicable to all contaminants. Phytoremediation may take longer than other technologies 

to treat a site, but it has the potential to be less 

expensive than excavating and treating large 

volumes of soil ex situ. 

There are a number of limitations to the technology 

that must be considered before it can be implemented at a site. The depth of the contamination 

requiring treatment must be within the range of 

depth of plant root growth; thus, treatment of 

contaminated soil typically focuses on the upper 8 

to10 inches of the soil horizon, although the roots of 

hybrid poplar trees, a species commonly used in 

phytoremediation, can grow to depths of about 15 

feet. Contaminants must be in contact with the root 

zone to be treated; therefore, a denser root mass is 

preferred to help contact more of the contamination. 

Because treatment depends on this contact with the 

root zone, phytoremediation is limited by the rate of 

root growth. Slower growth rates increase the time 

required to treat a site, and winter months may shut 

down the treatment system completely while plants 

are dormant. 

Another limitation of phytoremediation is possible 

bioconcentration of contaminants up the food chain. 

Several phytoremediation mechanisms work by 

incorporating the contaminant into the plant or 

holding it within the root zone. The contaminated 

vegetation and root zone may impact plant-eating 

20 
animals and soil organisms. This is particularly a 

concern with metals and radionuclide contamination 

that accumulate in plants and the root zone. Most 

plants do not accumulate significant levels of organic 

contaminants (U.S. EPA 2000); thus, bioconcentration is of less concern. The potential for planteating animals to be exposed is greatest when these 

contaminants accumulate in fruits, seeds, and leaves, 

so monitoring the fate of contaminants within the 

plants is important. To avoid bioconcentration in the 

food chain, contaminated plants can be harvested for 

disposal, destruction, or the extraction of metals for 

reuse (“phytomining”). In these cases, perimeter 

fencing and overhead netting can be installed to 

prevent animals from consuming contaminated plant 

matter. 

Finally, it is important to ensure that unwanted 

transfer of contaminants from soil to other media, 

such as the volatilization of organic compounds to the 

atmosphere through plant uptake and transpiration, 

does not occur or that the transfer results in the 

destruction of the contaminants. Evaluating the 

limitations of phytoremediation in its various applications, as well as assessing its potential effectiveness 

at contaminated sites, can be done in laboratory and 

field studies prior to implementation. Samples of site 

soil containing the target contaminants in a range of 

concentrations should be tested using the specific 

plants under consideration. Ultimately, sites undergoing phytoremediation must be monitored to assess 

the fate of contaminants. 

In general, phytoremediation has been implemented 

at a number of sites at full-scale (U.S. EPA 2006). 

Also, a wide range of site conditions, plants, and 

contaminants have been studied under laboratory or 

field testing. As of this publication, phytoemediation 

technologies have been selected 18 times by the 

Superfund program. 

Phytoextraction 

Phytoextraction involves the uptake of contaminants 

by plant roots, with subsequent accumulation in plant 

tissue, which may require that the plant be harvested 

and properly disposed of. This mechanism is typically 

used to treat inorganic contaminants, such as metals, 

metalloids, and radionuclides. Organic contaminants 

are more likely to be transformed, rather than 

accumulated, within the plant tissue. Successful field 

applications of phytoextraction to up take metals have 

been limited; however, there is some promising 

research for using phytoextraction on mercury and 

persistent organic pollutants. 

Plants used in phytoextraction include Indian 

mustard, pennycress, and alyssum sunflowers. They 

are typically effective only in the top one foot of soil 

because of their shallow root systems and generally 

slow growth. BaÁuelos et al. (2005) has shown that 

Indian mustard can be genetically modified to grow 

in contaminated soil with greater biomass to hyperaccumulate selenium in a shorter time than unmodified Indian mustard. In other genetic research, 

Meagher (undated) is modifying various plant 

species to survive in mercury-contaminated soil and 

to transform organic mercury into ionic and/or 

metallic mercury. The mercury is either sequestered 

in the plant or transpired. 

Persistent organic pollutants, many of which are 

pesticides, resist biodegradation and may remain in 

the environment for decades. White (2001) and 

Mattina (2000) have shown that a number of plants 

are capable of extracting chemicals, such as 

chlordane and 2,2-bis(p-chlorophenyl) 1,1-dichloroethene (p.p’-DDE), and storing them in their roots, 

leaves, and fruits. 

Rhizodegradation 

Rhizodegradation is essentially “plant-assisted bioremediation” in that the root zone enhances microbial activity, thus increasing the breakdown of 

organic contaminants (such as petroleum hydrocarbons, PAHs, pesticides, BTEX, chlorinated solvents, PCP, PCBs, and surfactants) in the soil. The 

term comes from “rhizosphere,” which is the zone of 

soil influenced by plant roots. This zone extends 

only about 1 mm from each root. 

The presence of plant roots increases soil aeration 

and moderates soil moisture, making conditions 

more favorable to bioremediation. Bioremediation is 

enhanced by the production of root exudates, such as 

sugars, amino acids, and other compounds, that can 

stimulate the population growth and activity of 

native microbes. Root exudates may also serve as 

food for the microbes, which can result in cometabolism of contaminants as degradation of exudates 

occurs. Because the microbes consume nutrients, the 

plants in a rhizodegradation plot often require 

additional fertilization. 

21 
The advantage of rhizodegradation is the actual 

breakdown of contaminants, rather than their translocation; thus, harvesting is not necessary. In some 

instances, complete mineralization of the contaminant can occur. Success, however, is site-specific, 

and laboratory microcosms may not reflect the 

microbial conditions encountered in the field. 

Petroleum hydrocarbons have been shown to be 

successfully degraded in the rhizosphere; however, 

degradation of aged hydrocarbons has been 

demonstrated to be more problematic. 

Phytodegradation 

Like phytoextraction, phytodegradation involves the 

uptake of contaminants; however, the contaminants 

are subsequently broken down through metabolic 

processes within the plant. Phytodegradation also 

comprises the breakdown of contaminants in the soil 

through the effects of enzymes and other compounds 

produced by the plant tissues (other than the roots). 

Phytodegradation is applicable to organic contaminants. Their uptake is affected by their hydrophobicity, solubility, and polarity. Moderately 

hydrophobic and polar compounds are more likely to 

be taken up after sorbing to plant roots (Schnoor et al. 

1995 and Bell 1992). Contaminants with the potential 

for phytodegradation include chlorinated solvents, 

herbicides, insecticides, PCP, PCBs, and munitions. 

Phytovolatilization 

Phytovolatilization is the uptake of a contaminant 

into a plant and its subsequent transpiration to the 

atmosphere, or the transformation or phytodegradation of the contaminant with subsequent transpiration of the transformation or degradation product(s) to the atmosphere. Phytovolatilization is more 

commonly applied to groundwater, but can also be 

applied to soluble soil contaminants. 

Phytovolatilization involving transformation or 

degradation of the contaminant has the advantage of 

potentially creating a less toxic product that is 

transpired; however, this also poses a potential 

drawback in that degradation of some contaminants, 

like TCE, may produce even more toxic products 

(e.g., vinyl chloride). This possibility has to be assessed on a site-specific basis, and measurement of 

transpired compounds can be difficult. Once in the 

atmosphere, these products may be more effectively 

degraded by sunlight (photodegradation) than they 

would be by the plant (phytodegradation). 

Both organic and inorganic contaminants have been 

treated by phytovolatilization. Inorganic contaminants include selenium, mercury, and arsenic; however, simply volatilizing a contaminant may not be 

an acceptable alternative. 

Phytostabilization 

Phytostabilization is a mechanism that immobilizes 

contaminants—mainly metals—within the root zone, 

limiting their migration. The contaminants are 

immobilized by adsorption of metals to plant roots, 

precipitation of metal ions (e.g., due to a change in 

pH), formation of metal complexes, or a change to a 

less toxic redox state. Phytostabilization can occur 

when plants alter the chemical and microbial 

makeup of the soil (e.g., through the production of 

exudates or carbon dioxide), which affects the fate 

and transport of the soil metals. Phytostabilization 

also encompasses the use of plants to prevent 

migration of soil contaminants with wind and water 

erosion, leaching, and soil dispersion. 

Since contaminants are retained in the soil, phytostabilization does not require the harvesting and 

disposal of plants. A phytostabilization system must 

be evaluated, however, to ensure that translocation 

of contaminants into the plant tissue is not 

occurring. Since contaminants remain in the root 

zone, the health of the plants must be maintained to 

prevent future release of contaminants when the 

plants die or are inadvertently destroyed. Maintenance may include the addition of fertilizers or soil 

amendments. 

Cited and Other References 

Bell, R.M. 1992. Higher Plant Accumulation of 

Organic Pollutants from Soils. EPA Office of 

Research and Development. Risk Reduction Engineering Laboratory, Cincinnati, Ohio. EPA 600/R92/138. 

BaÁuelos, G. et al. 2005. Field trial of transgenic 

Indian mustard plants shows enhanced phytoremediation of selenium-contaminated sediment. 

Environ. Sci. Technol. 39: 1771-1777. 

Brigmon, R. et al. 2003. FY02 Final Report on 

Phytoremediation of Chlorinated Ethenes in Southern Sector Seepline Sediments of the Savannah 

River Site. U.S. DOE, 171 pp. http://sti.srs.gov/full 

text/tr2002557/tr2002557.pdf 

22 
Ibeanusi, V. and D. Grab. 2004. Radionuclide 

Biological Remediation Resource Guide, EPA 905/B04/001. U.S. EPA, Region 5, 68 pp. http:// 

cluin.org/download/remed/905b04001.pdf 

ITRC. 2001. Phytotechnology Technical and 

Regulatory Guidance Document. Interstate Technology and Regulatory Council, 123 pp. http:// 

www.itrcweb.org/Documents/PHYTO-2.pdf 

Mattina, M., W. Iannucci-Berger, and L Dykas. 2000. 

Chlordane uptake and its translocation in food crops. 

Agric. Food Chem. 48(5): 1909-1915. 

Meagher, R. 2006 (October). Mercury Research. 

http://www.genetics.uga.edu/rbmlab/phyto/mer 

curypol.html 

Pivetz, B. 2001. Ground Water Issue: Phytoremediation of Contaminated Soil and Ground Water 

at Hazardous Waste Sites, EPA 540/S-01/500. U.S. 

EPA, Office of Research and Development, 36 pp. 

http://www.epa.gov/ada/download/issue/epa_ 

540_s01_500.pdf 

Remediation Technologies Development Forum. 

2006 (October). Evaluation of Phytoremediation for 

Management of Chlorinated Solvents in Soil and 

Groundwater. Phytoremediation of Organics Action 

Team, Chlorinated Solvents Workgroup, 42 pp. 

http://www.rtdf.org/public/phyto/chlor_solv_man 

agement.pdf 

Reynolds, C. 2004. Cost and Performance Report: 

Field Demonstration of Rhizosphere-Enhanced 

Treatment of Organics-Contaminated Soils on Native 

American Lands with Application to Northern FUD 

Sites, ERDC/CRREL LR-04-19. U.S. DoD, Environmental Security Technology Certification Program, 

53 pp. http://stinet.dtic.mil/oai/oai?&verb=getRecord 

&metadataPrefix=html&identifier=ADA431035 

Schnoor, J., L. Licht, S. McCutcheon, N. Wolfe, and 

L. Carreira, 1995. Phytoremediation of Organic and 

nutrient contaminants. Environ. Sci. Technol. 

29:318A-323A. 

Schnoor, J. 1997. Technology Evaluation Report: 

Phytoremediation, TE-98-01. Ground-Water Remediation Technologies Analysis Center, 43 pp. 

U.S. EPA. 2000. Introduction to Phytoremediation, 

EPA 600/R-99/107. Office of Research and Development, 104 pp. http://www.cluin.org/download/ 

remed/introphyto.pdf 

U.S. EPA. 2003. Innovative Technology Evaluation 

Report: Phytoremediation of Groundwater at Air 

Force Plant 4 Carswell, Texas, EPA 540/R-03/506. 

Office of Research and Development, 12 pp. http:// 

www.epa.gov/ORD/NRMRL/pubs/540r03506/ 

540R03506.pdf 

U.S. EPA. 2004. Treatment Technologies for Site 

Cleanup: Annual Status Report (Eleventh Edition). 

EPA 542/R-03/009. http://clu-in.org/asr 

U.S.EPA. 2006 (October). Phytoremediation Profile 

Database. http://www.cluin.org/products/phyto 

White, J. 2001. Plant-facilitated mobilization and 

translocation of weathered 2,2-bis(p-chlorophenyl) 

1,1-dichloroethene (p.p’-DDE) from agricultural 

soil. Environ. Toxicol. Chem. 20(9): 2047-2052. 

3.2.3 Monitored Natural Attenuation 

Monitored natural attenuation (MNA) encompasses 

the dilution, dispersion, chemical and biological 

degradation, sorption/precipitation, and/or radioactive decay of contaminants in soil and 

groundwater (U.S. EPA 1999). It has been applied 

mainly to groundwater contamination, but the same 

principles apply to soil. Because MNA is a passive 

process in which the reduction in contaminant 

concentration is due solely to natural mechanisms, 

continuous sources of significant contamination 

should be addressed before implementing MNA. If 

MNA is implemented, reaching remediation goals 

may take longer than other remedies. 

Site contaminants most amenable to MNA include 

petroleum hydrocarbons, low-molecular weight 

alcohols, ketones, esters, ethers, and iron and manganese. Under a narrow range of conditions, MNA 

may be applicable to halogenated solvents, lightly 

halogenated aromatics, polychlorinated biphenyls, 

nitroaromatics, some pesticides, and chromium, copper, cadmium, lead, zinc, and nickel (NRC 2000). 

A full characterization of subsurface conditions at 

the site, including a delineation of the extent of 

contamination and the development of a site 

conceptual model, are necessary before MNA can be 

23 
considered. As part of the characterization, the 

microbial species present (more important for 

chlorinated compounds), redox potential, pH 

conditions, mineralogy, and geochemistry should be 

evaluated. If the evaluation concludes that the 

conditions may exist to support natural attenuation, 

the next step is to determine if it is occurring. This is 

accomplished by looking at such parameters as 

microbial respiration products, chemical reaction 

products, prevalent metal species, degradation products, and declining concentrations of target compounds/species over time. For a more complete list 

and discussion, see Kram and Goetz 1999. If a large 

amount of historical data is not available to confirm 

that the contaminant mass is stabilized or contracting, 

it probably will be necessary to model the fate and 

transport of the contaminants to show that migration 

is unlikely to occur. 

Performance monitoring is an integral part of any 

MNA effort (Pope et al. 2004 and Wiedemeier, 

Lucas, and Haas 2000). The monitoring system 

should be tailored to site conditions to enable 

detection of any changes in the assumptions used to 

select MNA. Key parameters, such as degradation 

products or an increase in target metal concentration 

that would indicate mobilization, should be chosen 

along with an appropriate frequency. The frequency 

of sample collection is related to the uncertainties 

inherent in the site conceptual model and the consequences of failure. As a precaution, a preapproved 

site remedial contingency plan should be created that 

can be implemented if monitoring indicates MNA is 

not meeting the project’s performance goals. 

The Naval Facilities Engineering Command provides 

an estimate for implementing MNA of between 

$50,000 and $200,000 per acre. This cost does not 

include site characterization, which may be higher 

than that for a site not being considered for MNA. 

These cost estimates were developed for a 

groundwater scenario. 

Cited and Other References 

Brauner, J. et al. 2004. Implementing Monitored 

Natural Attenuation and Expediting Closure at FuelRelease Sites. Air Force Center for Environmental 

Excellence, 247 pp. http://stinet.dtic.mil/cgibin/Get 

TRDoc?AD=ADA426387&Location=U2&doc=Get 

TRDoc.pdf 

Chapelle, F. et al. 2003. Methodology for Estimating 

Times of Remediation Associated With Monitored 

Natural Attenuation, Water-Resources Investigations 

Report 03–4057. U.S. Geological Survey, 58 pp. 

http://pubs.usgs.gov/wri/wri034057/ 

Kennedy, L., J. Everett, and J. Gonzales. Undated. 

Aqueous and Mineral Intrinsic Bioremediation 

Assessment (AMIBA) Protocol. Air Force Center 

for Environmental Excellence, 284 pp. http://www. 

afcee.brooks.af.mil/products/techtrans/monitored 

naturalattenuation/amiba/AMIBA%20Protocol.PDF 

Kram, M. and F. Goetz. 1999. User's Guide: Natural 

Attenuation General Data Guide, UG-2035-ENV. 

NAVFAC, 38 pp. http://stinet.dtic.mil/cgi-bin/Get 

TRDoc?AD=A361097&Location=U2&doc=Get 

TRDoc.pdf 

Looney, B. and K. Vangelas. 2004. Compatibility of 

Alternative Chlorinated Solvent Source Treatment 

Strategies with Monitored Natural Attenuation, 

WSRC-MS-2004-00236. U.S. DOE, Savannah 

River, 22 pp. http://sti.srs.gov/fulltext/ms2004236 

/ms2004236.pdf 

NAVFAC. 1998. Technical Guidelines for 

Evaluating Monitored Natural Attenuation of 

Petroleum Hydrocarbons and Chlorinated Solvents 

in Ground Water at Naval and Marine Corps 

Facilities. 92 pp. http://enviro.nfesc.navy.mil/erb/erb 

_a/support/wrk_grp/artt/mna1198.pdf 

NRC. 2000. Natural Attenuation for Groundwater 

Remediation. National Academy of Sciences, 

National Research Council, 292 pp. http://books. 

nap.edu/catalog/9792.html 

Pope, D. et al. 2004. Performance Monitoring of 

MNA Remedies for VOCs in Ground Water, 

EPA/600/R-04/027, U.S. EPA, Office of Research 

and Development. http://www.epa.gov/ada/down 

load/reports/600R04027/600R04027_fm.pdf 

U.S. EPA. 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and 

Underground Storage Tank Sites, Directive 9200.4­

17P. Office of Solid Waste and Emergency 

Response, 41 pp. http://nepis.epa.gov/pubtitleOSW 

ER.htm 

Wiedemeier, T., M. Lucas, and P. Haas. 2000. 

Designing Monitoring Programs to Effectively 

24 
Evaluate the Performance of Natural Attenuation. 

U.S. DoD, Air Force Center for Environmental 

Excellence, 55 pp. http://www.afcee.brooks.af.mil/ 

products/techtrans/MonitoredNaturalAttenua 

tion/ltm.pdf 

Wilson, J., J. Cho, B. Wilson, and J. Vardy. 2000. 

Natural Attenuation of MTBE in the Subsurface 

under Methanogenic Conditions, EPA 600/R-00/006. 

U.S. EPA, Office of Research and Development, 59 

pp. http://www.epa.gov/ada/download/reports/mtbe 

report.pdf 

Wilson, J. et al. 2001. Evaluation of the Protocol for 

Natural Attenuation of Chlorinated Solvents: Case 

Study at the Twin Cities Army Ammunition Plant, 

EPA 600/R-01/025. U.S. EPA, Office of Research 

and Development, 49 pp. http://www.epa.gov/ada/ 

download/reports/epa_600_r01_025.pdf 

Wilson, J., P. Kaiser, and C. Adair. 2005. Monitored 

Natural Attenuation of MTBE as a Risk Management Option at Leaking Underground Storage Tank 

Sites, EPA 600/R-04/1790. U.S. EPA, Office of 

Research and Development, 89 pp. http://www.epa. 

gov/ada/download/reports/600R04179/600R04 

179.pdf 

3.3 Thermal Treatment Technologies 

Five technologies are grouped under the in situ 

thermal treatment classification: electrical resistance 

heating, steam injection and extraction, conductive 

heating, radio-frequency heating, and vitrification. 

With the exception of vitrification, all of these 

treatment technologies rely on the addition of heat to 

the soil to increase the removal efficiency of volatile 

and semivolatile contaminants. Vapor extraction is an 

integral part of these remediation systems to ensure 

the removal and treatment of mobilized contaminants. 

Liquid extraction is also used during steam injection, 

and sometimes with other thermal technologies when 

groundwater flow rates are high and/or when the 

contaminant being recovered is semivolatile. 

In situ vitrification is unique among the thermal 

technologies in that the temperatures used will vitrify 

soil. The stable glass that is formed by vitrification 

will immobilize any nonvolatile contaminants that are 

present, including metals and radioactive materials. 

Davis (1997) provides a general discussion of the 

effects of heat on chemical and physical properties of 

organic contaminants. Vaporization is the main 

mechanism used in these technologies to enhance 

the recovery of VOCs. Vapor pressures of organic 

compounds increase exponentially with temperature, 

causing significant redistribution to the vapor phase 

as the subsurface is heated. When a NAPL is 

present, the combined vapor pressure of the NAPL 

and water determine the boiling temperature, and 

co-boiling of the two liquids occurs at temperatures 

less than the boiling point of water. Thus, by raising 

the temperature of the subsurface above the 

co-boiling temperature, NAPL can be removed. 

Continued heating of the subsurface recovers 

contaminants from the dissolved and adsorbed 

phases as well. 

Increasing the temperature also decreases viscosity, 

increases solubility, and decreases adsorption, all of 

which aid in the recovery of VOCs and SVOCs. For 

some SVOC NAPLs, such as creosote, viscosity 

reduction may be an important mechanism for 

increased contaminant recovery (Davis 1997). 

Hydrolysis may play a role in the destruction of 

some contaminants (e.g., chlorinated methanes and 

ethanes) as the soil temperature approaches 100o

C; 

however, the breakdown products may be more 

recalcitrant than the original contaminants 

(Washington 1995). 

Care should be taken in designing the systems to 

ensure that all plumbing, including monitoring 

wells, are capable of withstanding high heat. In the 

presence of clay, vadose zone heating by resistivity, 

conductance, or radio frequency may result in some 

settlement of the treatment area due to the drying of 

the clay. 

Cited and Other References 

Davis, E. 1997. Ground Water Issue: How Heat Can 

Enhance In-situ Soil and Aquifer Remediation: 

Important Chemical Properties and Guidance on 

Choosing the Appropriate Technique, EPA 

540/S-97/502. U.S. EPA., Office of Research and 

Development, 18 pp. http://www.cluin.org/down 

load/remed/heatenh.pdf 

USACE. 2006. Interim Unified Facilities Criteria: In 

Situ Thermal Remediation, Hazardous, Toxic, and 

Radioactive Waste, UFC 3-280-05. U.S. DoD, 258 

pp. http://www.environmental.usace.army.mil/sve. 

htm 

25 
Washington, J. W. 1995. Hydrolysis rates of dissolved volatile organic compounds: Principles, 

temperature effects and literature review, Ground 

Water, 33(3):415-424. 

3.3.1 Electrical Resistance Heating 

Electrical resistance heating (ERH) involves passing 

electrical current through moisture in the soil between 

an array of electrodes. As the current flows through 

the moisture in soil pores, the resistance of the soil 

produces heat. Originally, ERH for remediation 

purposes was developed by DOE and Battelle using 

six-phase electricity and hexagonal electrode arrays, 

however, expansion of a six-phase hexagonal array 

may result in the creation of cold and hot spots within 

the target subsurface area. Three-phase power and 

triangular electrode arrays are generally more 

applicable to irregularly shaped cleanup areas for 

full-scale treatment and are commonly used today 

(Beyke and Fleming 2005). 

Electrodes can be thought of as wells that are 

equipped to deliver electric power at selected depths 

and also act as vapor recovery wells. When groundwater flow rates are high and/or a semivolatile NAPL 

is to be recovered, liquids can also be extracted 

(Beyke and Fleming 2002). Electrodes may be 

installed using conventional drilling rigs. Installation 

can also be done using horizontal or angular drilling 

techniques. Care should be taken to ensure that the 

potential for stray currents is accounted for in the 

design. 

ERH systems can be deployed to any depth and used 

in both the vadose and saturated zone. If the system 

is deployed only in the vadose zone, water should be 

added at the electrodes to maintain the moisture 

content and thus, the flow of electricity (U.S. EPA 

2004). 

The horizontal spacing between electrodes is usually 

between 14 and 24 feet (Beyke and Fleming 2005). 

The trade-off in distance is between the cost of 

installing more electrodes and heating the soil more 

quickly or installing fewer electrodes and heating the 

soil over a longer time. 

While heating all soil, ERH preferentially heats the 

more conductive silt and clay first. Temperatures over 

100o

C can be generated in the saturated zone, and 

these temperatures produce steam and steam 

stripping, which is especially beneficial for the silts 

and clays as contaminant movement in them is 

usually diffusion limited. 

Volatilization and steam stripping with SVE-capture 

are the predominant removal mechanisms for most 

contaminants using this technology. (Beyke and 

Fleming 2005). Soil with a high natural organic 

carbon content will slow or prevent the recovery of 

some organic contaminants. 

U.S. EPA (2004) provides remediation cost 

estimates of $32 and $73 per cubic yard at two fullscale ERH sites. Beyke and Fleming (2005) estimate 

that ERH costs $200,000, plus $40 to $70 per cubic 

yard. The technology is proven and has been used at 

a number of sites. The number of vendors offering 

the technology are limited. 

Cited and Other References 

Beyke, G. and D. Fleming. 2002. Enhanced removal 

of separate phase viscous fuel by electrical 

resistance heating and multi-phase extraction. 9th 

Annual International Petroleum Environmental 

Conference, October 22-25, 2002, Albuquerque, 

NM. http://ipec.utulsa.edu/Conf2002/beyke_flem 

ing_130.pdf 

Beyke, G. and D. Fleming. 2005. In situ thermal 

remediation of DNAPL and LNAPL using electrical 

resistance heating. Remediation,15(3): 5-22. http:// 

www.thermalrs.com/TRS_docs/ERH%20Rem 

ediation%20Journal%20article.pdf 

Butherus, M. et al. 2004. Project overview: Successful field-scale in situ thermal NAPL remediation. Third International Conference on Oxidation 

and Reduction Technologies for Soil and Groundwater San Diego, CA, October 24-28, 2004. 

http://www.lm.doe.gov/documents/3_pro_doc/8_ 

references/papers/2004_ORT_Conference_paper.pdf 

U.S. EPA. 2004. In Situ Thermal Treatment of 

Chlorinated Solvents: Fundamentals and Field 

Applications, EPA 542/R-04/010. Office of Solid 

Waste and Emergency Response, 145 pp. http:// 

www.cluin.org/download/remed/epa542r04010.pdf 

3.3.2 Steam Injection and Extraction 

Steam injection and extraction (also known as steam 

enhanced extraction [SEE]) involves injection of 

26 
steam into injection wells and the recovery of 

mobilized groundwater, contaminants, and vapor 

from the recovery wells. Initially, when steam is 

injected into the subsurface, it gives up its latent heat 

of vaporization to the soil. As the steam loses heat, it 

condenses into a hot water phase that moves radially 

into the soil and displaces air and water in front of it. 

Continued input of steam eventually causes the soil 

near the well to reach steam temperatures, creating a 

steam front that begins to propagate away from the 

well. This process creates a moving front consisting 

of ambient temperature water/air that is pushed by a 

variable temperature zone of warm to very hot water. 

The water in turn is pushed out by the pressure of the 

steam moving into the steam temperature zone. The 

movement of the ambient-temperature water may 

displace NAPLs, a process that is aided by viscosity 

reduction when the hot water reaches the NAPL. The 

arrival of the steam phase vaporizes compounds 

remaining as residual saturation or adsorbed to the 

soil. These vapors are transported to the leading edge 

of the steam zone where they condense, forming a 

contaminant condensate bank. The condensate bank 

may have a tendency to sink, and it is important in the 

design of the system to ensure its capture. Schmidt et 

al. (2002) and Kaslusky and Udell (2002) have found 

that co-injection of air with the steam helps prevent 

downward contaminant migration. 

Thus, contaminant removal occurs by several 

mechanisms. Which one is most important in a given 

situation depends on the volatility of the contaminant. 

For VOCs, vaporization and co-boiling are the most 

important recovery mechanisms, while for SVOCs, 

displacement as a NAPL and viscosity reduction may 

be more important. Recovery wells are used to 

capture both liquids and gases and transport them to 

a surface facility for treatment. 

The applicability of steam injection to a particular site 

is determined by the permeability of the soil, the 

depth at which the contaminants reside, and the type 

and degree of heterogeneity, as well as the contaminant type. The permeability of the soil must be high 

enough to allow sufficient steam to be injected to heat 

the entire source zone. Higher injection rates can be 

achieved by increasing the injection pressure; 

however, in general, pressures should not be higher 

than 1.65 pounds per square inch per meter of depth, 

or the overburden pressure will be exceeded, and 

fracturing to the surface can be expected (Davis, 

1998). Thus, shallow treatment areas are difficult to 

heat with steam, and collection of all the vapors 

generated may be challenging. An impermeable surface cover may help in this regard. 

Heterogeneity of the Subsurface and Soil Type 

The soil type affects the ability of the steam to 

remove contaminants in two ways. The permeability 

determines how fast a steam front can move into and 

through the soil. Low permeability soil may not 

allow steam to move through it at an economical rate 

or may require unsupportable pressures to do so. 

The other aspect of soil that affects contaminant 

removal is its reactivity with contaminants. Silica 

based sands are not particularly reactive, and 

contaminants can be removed easily. Smectite clays 

and soil rich in organic matter both have the ability 

to bind some organic compounds and prevent their 

full removal at steam temperatures. 

When there is heterogeneity, steam tends to channel 

to the more permeable layer, or in the case of 

discontinuous layers, by-pass the less permeable 

one. If this happens, heating in the less permeable 

soil usually is done by conductive methods. To fully 

heat low permeability zones by conduction, steam 

should be injected on both sides of the low 

permeability zone, and the zone should be less than 

about 10 feet in thickness. 

Injection and Extraction Well Placement and 

Operation 

The most effective design of steam injection and 

extraction systems is to surround the contaminated 

zone with four to six injection wells and to extract 

the contaminants from the center. If the area to be 

treated is large, repeating patterns of injection and 

extraction wells may be used to cover the entire area 

(Davis 1998). The propagation of the steam front is 

a balance between the injection well's ability to add 

heat and the surrounding soil's ability to absorb it. 

When the rate of heat loss to the soil surrounding the 

front equals the system's ability to input it, the steam 

front will stop growing. The extraction well needs to 

be closer to the injection well than this point of 

equilibrium. Distances between wells have been reported as close as 1.5 meters and as far apart as 18 

meters. 

The wells and couplings handle very hot streams of 

vapors and their construction is a concern. Plastics, 

such as PVC, are generally not appropriate, and steel 

27 
is typically used. Since the injection wells are both 

very hot and pressurized, couplings should be 

carefully designed and an appropriate sealant for the 

annulus should be chosen. Cement grouts used in 

conventional water well completion may not withstand the pressure, heat, and expansion/contraction of 

the well casing itself and could crack, causing the 

release of steam to the surface. 

A characteristic of steam treatment in a source zone 

under saturated conditions that needs to be considered is override. Override occurs when there is a 

difference in density between two fluids (such as that 

between steam and ambient temperature water). The 

resulting interface tends to move the steam out and 

up, causing the top of the steam front to be 

considerably further from the injection well than the 

bottom. This situation can lead to untreated spots near 

the bottom of the injection well. Override cannot be 

completely eliminated, but it can be minimized within 

the constraints of the site hydrogeology by using high 

injection rates. 

Pressure cycling of the steam injection is generally 

utilized as part of the system operation. Cycling is the 

process where, after breakthrough of steam at the 

extraction wells, the steam injection system is shut 

down while allowing the extraction process to 

continue. The loss of pressure thermodynamically 

destabilizes the system, forcing the temperature to 

drop to restore stability. The system loses heat by 

evaporation of residual moisture and the contaminants that are collected by the extraction wells. Davis 

(1998) and Davis et al. (2005) report on several studies where repeated cycling has resulted in increased 

contaminant concentrations in the extracted vapors. 

Steam is a well documented technology for addressing NAPL source zones in unconsolidated subsurfaces (Davis 1998). For example, over 150,000 

gallons of creosote were destroyed or removed by 

steam at the Southern California Edison Company 

pole yard in Visalia, California. Several small scale, 

short duration demonstrations of steam injection in 

fractured rock have shown the potential for this 

technology to be effective in fractured rock settings 

(e.g., Loring Air Force Base, Davis et al. 2005), but 

additional research and field demonstrations are 

needed to fully determine its effectiveness. Because 

of the sensitivity of steam flow to heterogeneous 

subsurface conditions, more site characterization 

efforts may be required than for other heating 

technologies. 

Another steam delivery system combines steam 

injection with an in situ auger mixing system. In this 

application, steam is applied through specially 

designed augers while the soil is being mixed. The 

steam strips the volatile contaminants from the 

moving soil and brings them to the surface, where 

they are captured in a shroud or bell device and 

transported to a treatment system. Moos (1998) 

describes the use of an auger system at Argonne 

National Laboratory to treat primarily chlorinated 

solvent contaminated clays. The system applied 

steam and hot air injection to bring 70 to 80 percent 

of the contaminants to the surface for treatment. 

Following the steam application, zero-valent iron 

was mixed into the soil by the augers as a polishing 

step. A similar steam system was used at the DOE 

Pinellas site for remediating chlorinated hydrocarbon hotspots. It successfully removed a large 

amount of the contamination, but it was hampered 

by an undersized offgas treatment system that 

probably prevented it from reaching site cleanup 

goals (Davis 1998). 

Cost data for steam injection and extraction are 

limited. U.S. DOE (2000) reported cleanup at 

approximately $39 per cubic yard, but their system 

also employed electrical heating. The technology is 

mature and well established; however, few vendors 

use it for environmental remediation. 

Cited and Other References 

Alberti, L., et al. 2001. Detailed Addition Remediation Planning with Steam Injection. Integrated 

Concept for Groundwater Remediation, 14 pp. 

http://umweltwirtschaft-uw.de/incore/doku/ 

Steam%20Injection%20Milano.pdf 

Davis, E. 1998. Steam Injection for Soil and Aquifer 

Remediation, EPA 540/S-97/505. U.S. EPA, Office 

of Research and Development, 16 pp. http://www. 

epa.gov/tio/tsp/download/steaminj.pdf 

Davis, E., et al. 2005. Steam Enhanced Remediation 

Research for DNAPL in Fractured Rock, Loring Air 

Force Base, Limestone, Maine, EPA 540/R-05/010. 

U.S. EPA Office of Research and Development. 

http://www.epa.gov/ORD/NRMRL/pubs/540r05 

010/540r05010.pdf 

Heron, G., S. Carroll, and S. Nielsen. 2005. 

Full-scale removal of DNAPL constituents using 

28 
steam-enhanced extraction and electrical resistance 

heating. Ground Water Monitoring & Remediation 

25(4): 97-107. http://www.ce.berkeley.edu/~sitar/ 

ce274/Thermal%20remediation.pdf 

Kaslusky, S.F. and K.S. Udell. 2002. A theoretical 

model of air and steam co-injection to prevent the 

downward migration of DNAPLs during steamenhanced extraction. Journal of Contaminant 

Hydrology 55:213-232. 

Leif, R. et al. 1998. In situ hydrothermal oxidative 

destruction of DNAPLs in a creosote contaminated 

site. The First International Conference on Remediation of Chlorinated and Recalcitrant Compounds 

Monterey, CA, May 18-21, 1998. http://www.llnl. 

gov/tid/lof/documents/pdf/233527.pdf 

Moos, L. 1998. Optimization of soil mixing 

technology through metallic iron addition. Tenth 

National Technology Information Exchange (TIE) 

Workshop, Willowbrook, Illinois, October 27-29, 

1998, 120 pp. http://www.osti.gov/energycitations/ 

servlets/purl/12366-7OlbU8/webviewable/12366.pdf 

Schmidt, R., J. Gudbjerg, T.O. Sonnenborg, and K.H. 

Jensen. 2002. Removal of NAPLs from the 

unsaturated zone using steam: Prevention of downward migration by injecting mixtures of steam and 

air. Journal of Contaminant Hydrology 55: 233-260. 

U.S. DOE. 2000. Innovative Technology Summary 

Report: Hydrous Pyrolysis Oxidation/Dynamic 

Underground Stripping, DOE EM-05/04, 26 pp. 

http://apps.em.doe.gov/OST/pubs/itsrs/itsr1519.pdf 

U.S. EPA. 2003a. Cost and Performance Report: 

Dynamic Underground Stripping-Hydrous Pyrolysis 

Oxidation at the Savannah River Site 321-M Solvent 

Storage Tank Area, Aiken, South Carolina. Office of 

Solid Waste and Emergency Response, 12 pp. 

http://costperformance.org/pdf/20030627_331.pdf 

U.S. EPA. 2003b. Cost and Performance Report: 

Steam Enhanced Extraction at the A.G. Communications Systems Site, Northlake, Illinois. Office 

of Solid Waste and Emergency Response, 10 pp. 

http://costperformance.org/pdf/20030625_326.pdf 

U.S. EPA. 2004. In Situ Thermal Treatment of 

Chlorinated Solvents: Fundamentals and Field 

Applications, EPA 542/R-04/010. Office of Solid 

Waste and Emergency Response, 145 pp. http:// 

www.cluin.org/download/remed/epa542r04010.pdf 

3.3.3 Conductive Heating 

Conductive heating uses either an array of vertical 

heater/vacuum wells or, when the treatment area is 

within about six inches of the ground surface, 

surface heater blankets. While it is feasible to deploy 

all the wells in a heater/vacuum mode, the typical 

deployment is to place six heater-only wells in a 

hexagonal shape with a heater vacuum well 

occupying the center of each hexagon. 

The wells can be installed using conventional 

drilling techniques or direct push. Heater wells are 

constructed of steel pipe with the base sealed. A 

resistive heating unit is lowered into the well and 

current is supplied. The heating element typically 

operates at temperatures between 540o

 and 815oC 

(Baker and Heron 2004). The steel pipe is heated by 

radiant energy and the soil surrounding it by thermal 

conductance. 

The vacuum well contains the same steel pipe and 

heating element components as a standard heater 

well, but it is placed within a larger screened well to 

which a vacuum can be applied. Heat propagates in 

a cylindrical fashion from the well outward. The 

heating is fairly even through all dry textures of soil. 

The hottest soil (typically 590o

C) is in the immediate 

vicinity of the wells, while the coolest soil is at the 

midpoint between wells. When the vacuum is 

applied to the center well, volatilized organics are 

pulled through the high-temperature soil, where 

some of the contaminants may be degraded (Baker 

and Heron 2004). The extracted vapors are transported to the surface for treatment. 

Well spacing is chosen based on contaminant type 

and depth, soil moisture content, the minimum 

required temperature between wells, and the time 

desired to reach that temperature (U.S. EPA 2004). 

SVOCs, including high boiling components, such as 

PAHs or PCBs, generally need a soil temperature of 

325o

C for adequate desorption, while VOCs require 

less heat (usually 100o

C) (Baker and Heron 2004). 

The ability to treat high-boiling contaminants at 

temperatures well below their boiling points is 

largely due to the significant increase in vapor 

pressures at the temperatures present and the 

relatively long residence time in a very hot 

29 
subsurface (Biershenk et al. 2004).The temperature 

requirements typically lead to well placement 

distances of 6 to 7.5 feet for the SVOCs and 12 to 20 

feet for the VOCs. As with electrical resistance 

heating, the closer the wells, the faster the desired 

temperatures are reached. 

Conductive heating operates best in unsaturated soil; 

however, it does find application in saturated soil 

with low hydraulic conductivity. As the temperature 

around the heater wells increases, the water evaporates and a “dry” zone is created that expands outward. At the leading edge of this cylindrical zone, 

steam is created, which further expands the zone. In 

low permeability soil, any replacement water that 

attempts to flow into the “dry” zone is quickly boiled 

off. In soil with high hydraulic conductivities, the 

influx of water to replace that boiling off may be 

sufficient to prevent the soil from exceeding the 

boiling point of water, and target temperatures may 

not be met. If the treatment area contains saturated 

high hydraulic conductivity soil, then a dewatering 

system should be considered, or Baker and Heron 

(2004) suggest using a steam system to control water 

influx, as well as sweeping the permeable areas. 

Drying soils, especially fine-grained silt and clay, at 

high temperatures can result in shrinkage and 

cracking that will promote the removal of organics 

contained within them (U.S. EPA 2004). 

If concentrated halogenated organics are the 

contaminants of concern, the system—both piping 

and treatment—must be designed to withstand highly 

corrosive conditions. 

Thermal conductance systems also can consume large 

quantities of power. At a site in Alhambra, California, 

the remediation had to be carried out in phases to 

avoid exceeding the capacity of the local power 

supplier (Biershenk et al. 2004). 

Vendor cost estimates cited range from $100 to $250 

per ton (NAVFAC 1999). TerraTherm has an 

exclusive license in the United States to offer this 

technology for remediation. 

Cited and Other References 

Baker, R. and M. Kuhlman. 2002. A description of 

the mechanisms of in-situ thermal destruction (ISTD) 

reactions. 2nd International Conf. on Oxidation and 

Reduction Technologies for Soil and Groundwater, 

ORTs-2, Toronto, Ontario, Canada, Nov. 17-21, 

2002, 10 pp. http://www.terratherm.com/resources/ 

TechPapers/ORTs-2%20Baker%20and%20 

Kuhlman.pdf 

Baker, R.S. and G. Heron. 2004. In-situ delivery of 

heat by thermal conduction and steam injection for 

improved DNAPL remediation. Proceedings of the 

4th International Conf. on Remediation of 

Chlorinated and Recalcitrant Compounds, Monterey, 

CA, May 24-27, 2004. Battelle, Columbus, OH. 

http://www.terratherm.com/resources/TechPapers/ 

Terratherm%20Delivery%20of%20Heat%20Paper 

%20-%20Monterey.pdf 

Bierschenk, J. et al. 2004. Full-scale phase 1a results 

of ISTD remediation at former Alhambra, California 

wood treatment site. Proceedings of the 4th 

International Conf. on Remediation of Chlorinated 

and Recalcitrant Compounds, Monterey, CA, May 

24-27, 2004. Battelle, Columbus, OH. http://www. 

terratherm.com/resources/TechPapers/Terratherm 

%20Paper%20499%20-%20Monterey.pdf 

Conley, D., K. Hansen, G. Stegemeier, H. Vinegar, 

F. Fossati, F. Carl, and H. Clough. 2000. In situ 

thermal desorption of refined petroleum 

hydrocarbons from saturated soil. In: G.D. 

Wickramanayake and A.R. Gavaskar (eds.) Physical 

and Thermal Technologies: Remediation of Chlorinated and Recalcitrant Compounds. Battelle Press, 

Columbus, OH, pp. 197-206. http://www.terra 

therm.com/resources/TechPapers/ConleyHydrocarbons.pdf 

NAVFAC. 1999. Cost and Performance Report: InSitu Thermal Desorption at the Former Mare Island 

Naval Shipyard, 21 pp. http://costperformance.org/ 

pdf/MareIsland.pdf 

U.S.EPA. 1997. Analysis of Selected Enhancements 

for Soil Vapor Extraction, EPA 542/R-97/007. 

Office of Solid Waste and Emergency Response, 

246 pp. http://www.cluin.org/download/remed/ 

sveenhmt.pdf 

U.S. EPA. 2004. Innovative Technology Evaluation: 

Field Evaluation of TerraTherm In Situ Thermal 

Destruction (ISTD) Treatment of Hexachlorocyclopentadiene, EPA 540/R-05/007. Office of Research 

and Development, 183 pp. http://www.epa.gov/ 

ORD/NRMRL/pubs/540r05007/540r05007.htm 

30 
3.3.4 Radio-Frequency Heating 

Radio-frequency heating (RFH) uses a high 

frequency alternating electric field for in situ heating 

of soils. The technique depends on the presence of 

dielectric materials with unevenly distributed 

electrical charges. The application of an electric field 

produces movement of the "polar" molecules, and 

this vibration creates mechanical heat. A spread of 

radio frequencies (e.g., 6.78 MHz, 13.56 MHz, 27.12 

MHz, and 40.68 MHz along with seven higher 

frequencies) regulated and assigned by the Federal 

Communications Commission (FCC) can be used in 

industrial, scientific, or medical applications. Under 

47 CFR18.301 (FCC 2006) an equipment designer or 

operator can use these frequencies without obtaining 

a license from the FCC. Higher frequencies increase 

the rate of heating but reduce the depth of penetration 

(Halliburton NUS Environmental 1995). 

A radio-frequency heating system usually consists of: 

• A three-phase power supply. 

• A radio-frequency source with an oscillator that 

generates a low-power current at the desired 

radio frequency, several serial amplifiers that 

increase the strength of the oscillator current, and 

a final amplifier that delivers the current at the 

prescribed output level. 

• An applicator system consisting of electrodes or 

antennae. 

• A monitoring control system. 

• A grounded metal shield over the treatment area. 

• A vapor collection and treatment system 

(Haliburton NUS Environmental 1995). 

Depending upon the vendor, a row or rows of 

applicator electrodes are placed in the ground to the 

depth of the treatment zone. The electrodes can be 

placed with conventional drilling equipment or direct 

push. In some designs, the electrodes themselves are 

used to recover soil gas and heated vapors. In other 

designs, wells are placed specifically for soil vapor 

extraction and to act as electromagnetic sinks to 

prevent heating beyond the treatment zone (Figure 5). 

Heating is both radiative and conductive, with soil 

near the applicator electrodes heating fastest (the 

radio frequency wave gets weaker the further from 

the electrode due to energy absorption). 

At the oscillation frequencies of the applied electromagnetic field, water generally acquires the largest 

dipole moment of any of the soil constituents before 

the polarity of the field reverses. As the water 

content falls, the heating relies on other polar parts 

of the soil. Careful consideration should be given to 

purely sandy soils as to whether RFH is applicable. 

Silica sand is non-polar, and heating in sand must 

rely on impurities present (Iben, Edelstein, and 

Roemer 1996). Also, the drier a soil becomes the 

more difficult it is to move organic gas through it. 

Conversely, too much water becomes a heat sink. In 

saturated conditions, RFH boils the water in the 

immediate vicinity of the applicator electrode and 

does not heat the treatment zone to a useful temperature. If the water table is shallow, dewatering 

techniques may need to be applied (Edelstein et al. 

1996 and Davis 1997). 

The antenna method places vapor recovery wells 

around the treatment area. Drilled or pushed 

applicator boreholes on site-specific spacing are 

lined with a fiberglass casing or other nonconductive 

non-polar material that will withstand the temperatures expected. The antenna are lowered into the 

applicator holes to an appropriate depth, and the 

heating is begun. The antenna can be lowered or 

raised as desired. 

Figure 5. Radio-Frequency Heating with Electrodes 

Both antenna and electrode systems monitor the heat 

distribution in the subsurface (usually with thermocouples, but other devices can be used) to ensure 

target temperatures are obtained throughout the 

treatment area. Depending upon the chemicals of 

concern, RFH can obtain temperatures over 250o

C 

with some vendors claiming temperatures to 400o

C 

(Davis 1997 and U.S. EPA 1997). These tempera31 
tures allow the system to treat both VOCs and many 

SVOCs. Also monitored is the impedance of the 

applicator system and the impedance of the subsurface soil. As the soil is heated, its impedance changes. 

If the applicator system impedance is not adjusted to 

match it, energy reflects back at the system, resulting 

in heating and potential failure. 

The vapor extraction system consists of conventional 

vapor extraction wells. Also, for safety and 

prevention of potential interference with local radio 

transmissions, a grounded metal shield is usually 

employed over, and just beyond, the treatment area. 

Metal structures are very efficient at absorbing RF 

energy and preventing it from escaping the treatment 

zone. For this reason, RFH is not applicable for 

treatment zones that contain metal or other 

conductive objects. Both antenna and electrode 

technologies were demonstrated by the EPA SITE 

program in 1994 (U.S. EPA 1995a & 1995b and 

Haliburton NUS Environmental 1995). 

Only the antenna system is currently offered, and that 

by one vendor. 

Cited and Other References 

Brody, K., A. Kabir, H. J. Cho, R. Fiacco, Jr., J. 

McTigue, M. Daly, and R. Kasevich. 2004. Implementing RF heating in fractured bedrock to remediate 

TCA DNAPL. 2004 Fractured Rock Conference: 

State of the Science and Measuring Success in 

Remediation, Portland, ME, September 13-15, 10 pp. 

http://clu-in.org/products/siteprof/2004fracrock 

conf/cdr_pdfs/indexed/group1/034.pdf 

Davis, E. 1997. Ground Water Issue: How Heat Can 

Enhance In-Situ Soil and Aquifer Remediation: 

Important Chemical Properties and Guidance on 

Choosing the Appropriate Technique, EPA 

540/S-97/502. U.S. EPA., Office of Research and 

Development, 18 pp. http://www.cluin.org/down 

load/remed/heatenh.pdf 

Edelstein, W. et al. 1996. Radiofrequency Ground 

Heating System for Soil Remediation, Patent Number 

5,484,985. General Electric Company. http:// 

patft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO1& 

Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2 

FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1= 

5,484,985.PN.&OS=PN/5,484,985&RS=PN/5,484, 

FCC. 2006. Code of Federal Regulations: Title 

47–Telecommunication Chapter I–Federal Communications Commission Part 18–Industrial, Scientific, and Medical Equipment. http://www.access. 

gpo.gov/nara/cfr/waisidx_05/47cfr18_05.html 

Haliburton NUS Environmental Corporation. 1995. 

Installation Restoration Program Technical Evaluation Report for the Demonstration of Radio Frequency Soil Decontamination at Site S-1. U.S. Air 

Force Center for Environmental Excellence, 1,298 

pp. http://www.osti.gov/bridge/purl.cover.jsp?purl 

=/146796-ze58o8/webviewable/ 

Iben, I., W. Edelstein, and P. Roemer. 1996. 

Technical Information Series: Dielectric Properties 

of Soil: Application to Radio Frequency Ground 

Heating, 96CRD150. GE Research and 

Development Center, 37 pp. http://www.crd.ge.com/ 

cooltechnologies/pdf/1996crd150.pdf 

U.S. EPA. 1995a. Innovative Technology 

Evaluation Report: IITRI Radio Frequency Heating 

Technology, EPA 540/R-94/527. Office of Research 

and Development, 162 pp. http://nepis.epa.gov/pub 

titleOSWER.htm 

U.S. EPA. 1995b. Innovative Technology 

Evaluation Report: Radio Frequency Heating, KAI 

Technologies, Inc, EPA 540/R-94/528. Office of 

Research and Development, 168 pp. http://nepis. 

epa.gov/pubtitleOSWER.htm 

U.S.EPA. 1997. Analysis of Selected Enhancements 

for Soil Vapor Extraction, EPA 542/R-97/007. 

Office of Solid Waste and Emergency Response, 

246 pp. http://www.cluin.org/download/remed/ 

sveenhmt.pdf 

3.3.5 In Situ Vitrification 

In situ vitrification (ISV) is a thermal treatment process that converts contaminated soil to stable glass 

and crystalline solids. There are two methods for 

producing heat for melting the contaminated soil. 

The older method uses electrodes and electrical 

resistance to vitrify materials, while the emerging 

technique uses plasma arc technology. 

In the electrical resistance method, high voltage is 

applied to electrodes (typically four) placed in the 

soil. Starter frit (generally graphite) is placed on the 

soil surface and electrical current heats the soil from 

the top down to temperatures between 1,400 and 

32 

985 
2,000°C. Typical melt sizes range from 200 to 1,200 

tons, with a processing rate of four to six tons per 

hour (U.S. EPA 1995). Maximum treatment depth is 

approximately 20 feet in a single setup. The process 

depends upon the presence of 1.4 to 15 percent alkali 

metal oxides in the material to be treated to ensure a 

proper balance between electrical conductivity and 

melting temperature. Too much alkali metal content 

increases the conductivity to a point where 

insufficient heating occurs. 

If the silica content of the soil is sufficiently high, 

contaminated soil can be converted into glass. 

Heating vaporizes or pyrolyzes organic contaminants. 

Most inorganic contaminants are encased in the glasslike monolith that results when the soil cools after 

treatment. The system requires a vapor hood that 

traps offgases and channels them to a treatment train 

that generally consists of a quencher to cool the 100° 

to 400°C gases and, depending upon what is being 

treated, a scrubber, activated carbon unit, or thermal 

oxidizer (U.S. EPA 1997a). The scrubber and quench 

water may require secondary treatment. 

The conventional ISV process can destroy or remove 

organics and immobilize most inorganics in contaminated soil, sludge, or other earthen materials The 

process has been used on a broad range of VOCs and 

SVOCs, other organics including dioxins and PCBs, 

and on most priority pollutant metals and 

radionuclides ( http://www.frtr.gov/). 

Mercury is not captured in the 

vitrified soil and therefore must be 

treated by the above ground 

treatment system. The majority (70 

to 99.9 percent by weight) of heavy 

metals, such as arsenic, lead, 

cadmium, and chromium, and 

radionuclides are entrained in the 

melt mass (Thompson, Bates and 

Hansen 1992). SVOCs and VOCs 

can be treated with this process, with 

about 97 percent of the VOCs 

destroyed and the remainder captured 

by the offgas treatment system (U.S. 

EPA 1997a). ISV is applicable to Figure 6. Planar Vitrification—Melting from the Bottom 

ISV, using the electrical resistance method, has been 

tested in the field several times, including a SITE 

Program demonstration (U.S. EPA 1995), a 

demonstration at the DOE Hanford Nuclear 

Reservation (http://www.frtr.gov/) and Superfund 

cleanups at the Wasatch Chemical Company, Lot 6 

site (U.S. EPA 1997b) and General Electric Spokane 

Shop (U.S. EPA 2005). Costs are estimated at 

$400/ton (U.S. EPA 1997a). The technology is 

licensed to only one vendor. 

Planar melting is a modification of the conventional 

ISV method. It differs in that the starter material is 

injected in a vertical plane between electrodes at 

depth. Generally, two electrode pairs are used with 

a starter plane between each pair. As the melt 

proceeds, it grows vertically and horizontally away 

from the starter planes. Because the melts are 

initially separated and only merge late in the 

process, the potential for driving gases down into the 

formation is greatly reduced as compared with 

conventional ISV (Figure 6). The maximum established treatment depth is 26 feet, but deeper melts 

are theoretically possible. The cost of the process is 

estimated at between $355 and $460 per ton 

(Thompson 2002). A successful field demonstration 

of the planar technique was carried out at the Los 

Alamos National Laboratory in 2000 (Coel-Roback 

et al. 2003). 

sites with high clay and moisture 

content, although treatment costs increase with increasing moisture content. Treatment of materials in 

a permeable aquifer may require dewatering, and if 

the treatment area is expected to contain large voids, 

dynamic compaction is recommended (U.S. EPA 

1997a). 

In situ vitrification using plasma arc technology has 

been demonstrated at the DOE Savannah River 

Complex (Blundy and Zionkowski 1997) but has yet 

to reach commercialization. The process consists of 

lowering a plasma arc torch into a cased hole and 

initiating a columnar melt from the bottom up. 

33 
Offgases are collected in a hood and treated by a 

system similar to the electrode method. The torch can 

achieve temperatures exceeding 7,000o

C, and theoretically, it can operate at any depth. Melts can be 

initiated below the contaminated area to ensure all 

contamination is addressed. Since a full-scale demonstration has yet to be done, cost data are unavailable. 

Cited and Other References 

Blundy, R. and P. Zionkowski. 1997. Final Report: 

Demonstration of Plasma In Situ Vitrification at the 

904-65G K-Reactor Seepage Basin, WSRC-RP-97­

405. Westinghouse Savannah River Company, 192 

pp. http://www.osti.gov/bridge/purl.cover.jsp?purl= 

/626442-vswIVz/webviewable/ 

Coel-Roback, B., P. Lowery, M. Springer, L. 

Thompson, and G. Huddleston. 2003. Non-traditional 

in situ vitrification—A technology demonstration at 

Los Alamos National Laboratory. WM’03 

Conference, February 23 – 27, 2003, Tucson, AZ, 12 

pp. http://www.wmsym.org/abstracts/2003/pdfs/ 

460.pdf 

FRTR. 2006 (November). Federal Remediation 

Technologies Roundtable website: http://www.frtr. 

gov//matrix2/section4/4-8.html 

Schumacher, R.F. 1996. Georgia Tech Final Report: 

Demonstration In Situ Plasma Vitrification 

Technology for Savannah River Site Contaminated 

Soils. U.S. Department of Energy, 147 pp. 

http://www.osti.gov/bridge/product.biblio.jsp? 

osti_id=656899 

Thompson, L., S. Bates, and J. Hansen. 1992. 

Technology Status Report: In Situ Vitrification 

Applied to Buried Wastes, PNL-8219. Pacific 

Northwest Laboratory. 

Thompson, L. 2002. Mixed waste treatment cost 

analyses for a range of Geomelt vitrification process 

configurations. WM'02 Conference, February 24-28, 

2002, Tucson, AZ. 13 pp. http://www.wmsym.org/ 

Abstracts/2002/Proceedings/39B/323.pdf 

U.S. EPA. 1994a. Engineering Bulletin: In Situ 

Vitrification Treatment, EPA 540/S-94/504. Office of 

Emergency and Remedial Response, 8 pp. http:// 

nepis.epa.gov/pubtitleOSWER.htm 

U.S. EPA. 1994b. SITE Demonstration Bulletin: In 

Situ Vitrification Geosafe Corporation, EPA 540/ 

MR-94/520. Office of Research and Development, 

2 pp. http://nepis.epa.gov/pubtitleOSWER.htm 

U.S. EPA. 1995. Geosafe Corporation In Situ 

Vitrification Innovative Technology Evaluation 

Report, EPA 540/R-94/520. Office of Research and 

Development, 148 pp. http://www.epa.gov/ORD/ 

SITE/reports/540r94520/540r94520.htm 

U.S. EPA. 1997a. Innovative Site Remediation 

Design and Application, Volume 4: Stabilization/ 

Solidification, EPA 542/B-97/007. Office of Solid 

Waste and Emergency Response, 234 pp. http:// 

nepisepa.gov/pubtitleOSWER.htm 

U.S. EPA. 1997b. Wasatch Chemical Company 

Superfund Site: Five-Year Review Report. http:// 

cfpub.epa.gov/fiveyear/index.cfm?fuseaction= 

home.showSearchForm 

U.S. EPA. 2005. NPL Sites in Region 10. http:// 

yosemite.epa.gov/r10/cleanup.nsf/webpage/ 

Washington+Cleanup+Sites 

4.0 Notice, Disclaimer, and Acknowledgments 

This document has been prepared for the U.S. 

Environmental Protection Agency under Contract 

Number 68-W-03-038. This report has undergone 

EPA review. Mention of trade names or commercial 

products does not constitute endorsement or 

recommendation for use. 

Special acknowledgement is given to the remediation professionals for their review and thoughtful 

suggestions to support the preparation of this document, including the following: Carolyn Acheson 

(EPA ORD), Frank Bales (USACE), Harold Ball 

(EPA Region 9), Ed Barth (EPA ORD), Ed Bates 

(EPA ORD), Dave Becker (USACE), Terry Burton 

(EPA ORD), Jim Cummings (EPA OSWER), Eva 

Davis (EPA ORD), Carol Dona (USACE), Ed 

Feltcorn (EPA OAR), Robert Ford (EPA ORD), 

Michael Gill (EPA ORD), Richard Griffith (EPA 

ORD), Scott Huling (EPA ORD), Steve Kinser 

(EPA Region 7), Gene Keepper (EPA Region 6), 

Lindsey Lein (USACE), Kelly Madalinski (EPA 

OSWER), Ed Mead (USACE), Randy Parker (EPA 

ORD), David Reisman (EPA ORD), Marta Richards 

34 
(EPA ORD), Hugh Rieck (USACE), Steve Rock 

(EPA ORD), Bill Rothenmeyer (EPA Region 8), 

Ellen Rubin (EPA OSWER), Michelle Simon (EPA 

ORD), and Stuart Walker (EPA OSWER). 

5.0 Abbreviations and Acronyms 

Air Force Center for Environmental 

Excellence 

benzene, toluene, ethyl benzene, xylene 

centigrade 

chlorobenzenes 

dichloromethane or methylene chloride 

trichloromethane or chloroform 

chromium 

carbon tetrachloride 

chlorinated volatile organic compound 

dichloroethane 

dichloroethene 

dichlorodiphenyltrichloroethane 

dense nonaqueous phase liquid 

direct push 

electrical resistance heating 

Federal Communications Commission 

Federal Remediation Technologies 

Roundtable 

hydrogen gas 

in situ vitrification 

monitored natural attenuation 

multi-phase extraction 

methyl tert-butyl ether 

nonaqueous phase liquid 

National Academy of Sciences 

AFCEE 

BTEX 

C 

CB 

CH2Cl2 

CHCl3 

Cr 

CT 

CVOC 

DCA 

DCE 

DDT 

DNAPL 

DP 

ERH 

FCC 

FRTR 

H2 

ISV 

MNA 

MPE 

MTBE 

NAPL 

NAS 

NAVFAC Naval Facilities Engineering Command 

NPL National Priority List 

OAR Office of Air and Radiation 

OH- hydroxide 

ORD Office of Research and Development 

OSWER Office of Solid Waste and Emergency 

Response 

PAHs polycyclic aromatic hydrocarbons 

PCB polychlorinated biphenyl 

PCE perchloroethene or tetrachloroethene 

PCP pentachlorophenol 

ppm parts per million 

PVC polyvinyl chloride 

RFH radio-frequency heating 

SCM site conceptual model 

SEAR surfactant-enhanced aquifer remediation 

SEE steam enhanced extraction 

SITE Superfund Innovative Technology 

Evaluation 

SOD soil oxidant demand 

S/S solidification/stabilization 

SVOCs semivolatile organic compounds 

SVE soil vapor extraction 

TCA trichloroethane 

TCE trichloroethene 

TCLP Toxicity Characteristic Leaching Procedure 

USACE United States Army Corps of Engineers 

U.S. DoD United States Department of Defense 

U.S. DOE United States Department of Energy 

U.S. EPA United States Environmental Protection 

Agency 

VC vinyl chloride 

VOCs volatile organic compounds 

35