Several thermal treatment methods or combinations of methods can be used to apply heat to polluted soil and/or groundwater in situ to destroy or volatilize organic chemicals. The chemical vapors are more mobile, and can be collected via extraction wells for cleanup in an ex situ treatment unit. Thermal methods can be particularly useful for dense or light nonaqueous phase liquids (DNAPLs or LNAPLs).

The main advantage of in situ thermal methods is they allow soil to be treated without excavating or transporting it, which saves money. However, in situ treatment generally requires more time than ex situ treatment, and uniformity of treatment is less certain because of the variability in soil and aquifer characteristics and because the efficacy of the process is more difficult to verify.

Heat can be introduced to the subsurface by electrical resistance heating; injection of hot water, hot air, or steam; radio frequency heating, dynamic underground stripping, thermal conduction, or vitrification.

ELECTRICAL RESISTANCE HEATING uses arrays of electrodes installed around a central neutral electrode to create a concentrated flow of current toward the central point. Resistance to flow in the soils generates heat greater than 100°C, producing steam and readily mobile contaminants that are recovered via vacuum extraction and processed at the surface. Electrical resistance heating is an extremely rapid form of remediation with case studies of effective treatment of soil and groundwater in less than 40 days. Three-phase heating and six-phase soil heating are varieties of this technology.

INJECTION OF HOT AIR can volatilize organic contaminants (e.g., fuel hydrocarbons) in soil or sediment. With deeper subsurface applications, hot air is introduced at high pressure through wells or soil fractures. In surface soils, hot air is usually applied in combination with soil mixing or tilling.

INJECTION OF HOT WATER via injection wells heats the soil and ground water and enhances contaminant release. Hot water injection also displaces fluids (including LNAPL and DNAPL free product) and decreases contaminant viscosity in the subsurface to accelerate remediation through enhanced recovery.

INJECTION OF STEAM heats the soil and groundwater and enhances the release of contaminants from the soil matrix by decreasing viscosity and accelerating volatilization. Steam injection may also destroy some contaminants. As steam is injected through a series of wells within and around a source area, the steam zone grows radially around each injection well. The steam front drives the contamination to a system of ground-water pumping wells in the saturated zone and soil vapor extraction wells in the vadose zone.

RADIO FREQUENCY HEATING uses electromagnetic energy to heat soil and enhance soil vapor extraction. The technique heats a discrete volume of soil using rows of vertical electrodes embedded in soil or other media. Heated soil volumes are bounded by two rows of ground electrodes with energy applied to a third row midway between the ground rows. The three rows act as a buried triplate capacitor. When energy is applied to the electrode array, heating begins at the top center and proceeds downward and outward through the soil volume. The technique can heat soil to over 300°C.

THERMAL CONDUCTION (also referred to as electrical conductive heating or in situ thermal desorption) supplies heat to the soil through wells where contamination is deep or with a blanket that covers the ground surface where contamination is shallow. Typically, a carrier gas or vacuum system transports the volatilized water and organics to a treatment system.

VITRIFICATION uses an electric current to melt contaminated soil at temperatures of 1,600 to 2,000°C (2,900 to 3,650°F). Upon cooling, the vitrified product is a chemically stable, leach-resistant, glass and crystalline material similar to obsidian or basalt. The high temperature component of the process destroys or removes organic materials. Radionuclides and heavy metals are retained within the vitrified product. Vitrification can be conducted in situ or ex situ.

Community Guide to In Situ Thermal Treatment
EPA 542-F-21-016, 2021

The Community Guide series (formerly Citizen's Guides) is a set of two-page fact sheets describing cleanup methods used at Superfund and other hazardous waste cleanup sites. Each guide answers six questions about the method: 1) What is it? 2) How does it work? 3) How long will it take? 4) Is it safe? 5) How might it affect me? 6) Why use it?

Critical Evaluation of State-of-the-Art In Situ Thermal Treatment Technologies for DNAPL Source Zone Treatment
J.T. Kingston, P.R. Dahlen, P.C. Johnson, E. Foote, and S. Williams.
ESTCP Project ER-0314, 1,272 pp, 2010

The performance of thermal technologies for DNAPL source zone remediation was assessed with particular emphasis on post-treatment groundwater quality and mass discharge (i.e., mass flux). Documents from 182 applications were collected and reviewed (87 electrical resistance heating, 46 steam-based heating, 26 conductive heating, and 23 other heating technology applications) conducted between 1988 and 2007, with attention to the site geologic settings, chemicals treated, design parameters, operating conditions, and performance metrics. The results of the study are summarized in a set of spreadsheet-based summary tables linking this information to five generalized geologic scenarios. The Summary Tables identify generalized scenarios that can be used to anticipate the likely performance of thermal-based DNAPL treatment technologies at a site. Another product of this work, 'State-of-the-Practice Overview of the Use of In Situ Thermal Technologies for NAPL Source Zone Cleanup,' condenses the 1,000-plus pages of this report into an 86-page primer prepared for a program manager audience. State-of-the-Practice Overview; ESTCP Cost & Performance Report

Engineering Paper: In Situ Thermal Treatment Technologies: Lessons Learned
2014

This paper conveys useful information gained from approximately 10 years of development and deployment of in situ thermal treatment (ISTT) technologies. It was compiled from a series of in‐depth interviews with U.S. EPA Remedial Project Managers (RPMs) and On‐Scene Coordinators (OSCs) and with ISTT vendors whose experience extends beyond federal response action sites to includestate‐regulated cleanups and Brownfields/voluntary cleanups, as well as international projects. While the focus is on federally funded cleanup sites, many of the lessons learned will be of interest to RPMs and OSCs who are overseeing potentially responsible party (PRP)-lead cleanups.

In Situ Thermal Remediation for Source Areas: Technology Advances and a Review of The Market From 1988-2020
Horst, J., J. Munholland, P. Hegele, M. Klemmer, and J. Gattenby. Groundwater Monitoring & Remediation [Published online 24 January 2021 prior to print]

The review picks up from the conclusion of a previous comprehensive review on in situ thermal remediation to discuss vendor evolution, the growth of the market for ISTR, related application trends, advancements, and thoughts on where the ISTR market may be headed in the next 10 years. The review includes a fresh look at the sustainability and resilience profile for ISTR.

Remediation Technologies Screening Matrix and Reference Guide, Version 4.0.
Federal Remediation Technologies Roundtable.

Thermal Treatment of Hydrocarbon-Impacted Soils: A Review of Technology Innovation for Sustainable Remediation
Vidonish, J.E., K. Zygourakis, C.A. Masiello, G. Sabadell, and P.J.J. Alvarez.
Engineering 2(4):426-437(2016)

The authors review thermal treatment technologies for hydrocarbon-contaminated soils, assess their potential environmental impacts, and propose frameworks for sustainable and low-impact deployment based on a holistic consideration of energy and water requirements, ecosystem ecology, and soil science. The review covers thermal desorption in situ and ex situ, smoldering, incineration, pyrolysis, vitrification, radio-frequency heating/microwave heating, hot air injection, and steam injection. Selecting an appropriate thermal treatment depends on the contamination scenario (including the type of hydrocarbons present) and on site-specific considerations such as soil properties, water availability, and the heat sensitivity of contaminated soils.