Dense nonaqueous phase liquids (dnapls)

Thermal Processes: In Situ

This page identifies general resources that contain information on the design and implementation of thermal treatment technologies applied in situ. Information on applications of these technologies specific to compounds within a DNAPL chemical class can be found in the class subsections listed to the right. More resources on thermal treatment technologies for a wide range of contaminants can be found in the Thermal Treatment: In Situ pages of Technology Focus.

In situ thermal treatment technologies can be applied to contaminant source zones. The application of heat increases the partitioning of organic chemicals into the vapor or gas phase, where they can be extracted under vacuum. The contaminants may even be destroyed in situ when temperatures are sufficiently high.

Four technologies are grouped under the in situ thermal treatment technologies classification: electrical resistance heating, steam injection and extraction, conductive 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 determines 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 vaporized and 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 100°C; however, the breakdown products may be more recalcitrant than the original contaminants.

Care should be taken in designing thermal treatment 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.

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This discussion is taken from Engineering Forum Issue Paper: In Situ Treatment Technologies for Contaminated Soil, EPA 542-F-06-013, 2006.

General Resources

Analysis of Selected Enhancements for Soil Vapor Extraction
U.S. EPA, EPA 542-R-97-007, 246 pp, 1997

This comprehensive survey on soil vapor extraction enhancement technologies includes a chapter on thermal enhancements that covers steam, resistive, and conductive heating. It provides a general discussion on these technologies with several case studies and some dated costing information.

Assessment of a Large Scale In-Situ Thermal Treatment Project Performed at a Chlorinated Solvent Site in the UK
Baldock, J., L. Chesher, D. Reid, A.-M. Sexton, A. Thomas, S. Tillotson, R. Niven, K. Johnson, and J. Dablow.
AquaConsoil 2013, 16-19 April 2013, Barcelona. Paper 1954, 7 pp, 2013

At an active UK manufacturing facility, the main mass of chlorinated solvent (mainly cis-1,2-DCE and VC) contamination lay within the matrix of underlying saturated, confined, and fractured bedrock. The remedial design combined 23 steam injection locations and 20 dual-phase vacuum extraction (DPVE) wells. Although a relatively energy-intensive remediation approach, thermal treatment was identified as the most rapid strategy for actively removing mass from bedrock. The total mass removed was calculated at ~1,000 kg. Asymptotic conditions with respect to mass removal were achieved within 14 weeks. Steam injection processes were monitored via a network of thermocouples and interpreted using PC-based software. DPVE performance also was assessed via regular flow and VOC quantification. A preliminary assessment indicated that the thermal remediation carbon footprint was 1,611 tonnes CO₂-equivalent to remove ~1,000 kg of mass.

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

Design: In Situ Thermal Remediation
U.S. Army Corps of Engineers. EM 200-1-21, 243 pp, 2014

This document provides guidance and background for the appropriate screening and selection of in situ thermal remediation technologies, including steam enhanced extraction/injection, electrical resistivity heating, and thermal conductive heating. The document is intended to help distinguish proper applications of the technology and identify important design, operational, and monitoring issues.

Effects of Thermal Treatments on the Chemical Reactivity of Trichloroethylene
J. Costanza, J. Mulholland, and K. Pennell.
EPA 600-R07-091, 117 pp, 2007

During experiments conducted to investigate abiotic degradation and reaction product formation of TCE when heated, the amounts of TCE degraded were very small at 120°C (0.01%) and 240°C (6.5%); however, a temperature of 420°C converted as much as 20% of the TCE to carbon dioxide and carbon monoxide.

Ground Water Issue: How Heat Can Enhance In-Situ Soil and Aquifer Remediation: Important Chemical Properties and Guidance on Choosing the Appropriate Technique
E. Davis.
U.S. EPA, EPA 540-S-97-502, 18 pp, 1997
Contact: Eva L. Davis, davis.eva@epa.gov

Contains in-depth information on the properties of some common organic contaminants (including DNAPLs) that can affect their movement in and recovery from the subsurface, as well as information on how these properties are affected by temperature. Basic information on which of the heat-based remediation techniques is most appropriate to certain subsurface conditions and certain contaminants is also provided, as well as a comparison of the heat-based techniques to other in situ remediation techniques. Three companion issue papers have been written to provide an explanation of how each of the three general types of thermal processes (steam or hot air injection, electrical heating, and hot water injection) works, as well as preliminary information on the design of a system and some estimates of the expected costs.

How Effective Is Thermal Remediation of DNAPL Source Zones in Reducing Groundwater Concentrations?
Baker, R.S., S.G. Nielsen, G. Heron, and N. Ploug.
Groundwater Monitoring & Remediation 36(1):38-53(2016)

Evaluation of data from 10 separate DNAPL source areas at 5 in situ thermal remediation project sites indicates that a thorough implementation of ISTR in a DNAPL source area can result in the attenuation of the associated dissolved plume, such that in several cases long-standing P&T systems could be turned off. These findings contrast with assertions that aggressive source remediation may not be justifiable because dissolved plume concentrations will not decline sufficiently.

Improved Monitoring Methods for Performance Assessment During Remediation of DNAPL Source Zones
R. Siegrist, R. Oesterreich, L. Woods, and M. Crimi.
Strategic Environmental Research and Development Program (SERDP), Project ER-1490, 116 pp, 2010

Investigators evaluated (1) the effects that sampling methods can have on the accuracy of measurements made for chlorinated solvents in samples of porous media collected from intact cores, and (2) the effects that remediation agents can have on the ability to infer CVOC mass levels in the subsurface based on groundwater concentration data. The accuracy of VOC measurements was investigated using an experimental apparatus packed with sandy porous media and contaminated with known levels of VOCs (PCE, TCE, TCA) sampled using different methods under variable, but controlled, conditions. Five sampling methods were examined representing different degrees of porous media disaggregation and duration of atmospheric exposure that can occur during sample acquisition and preservation in the field. CVOCs were studied at dissolved, sorbed, and nonaqueous phases. Five porous media temperatures were examined ranging from 5 to 80 degrees C to represent ambient or thermal remediation conditions, and two water saturation levels were used to mimic vadose zone and groundwater zone conditions. Results show that sampling method attributes can impact the accuracy of VOC measurements in porous media by causing negative bias in VOC concentration data ranging from near 0 to 90% or more. In situ remediation technologies, such as thermal treatment, ISCO, and flushing, have the potential to alter subsurface properties, which can affect the behavior of CVOCs, including DNAPLs.

In Situ Thermal Treatment Site Profile Database
U.S. EPA.

EPA has developed a Web site to summarize information about field demonstrations and full-scale applications of in situ thermal technologies. The searchable database provides project information consisting of site name, location, chemicals of concern, amount treated, costs (if available), and points of contact.

White Paper on Thermal Remediation Technologies for Treatment of Chlorinated Solvents: Santa Susana Field Laboratory, Simi Valley, California
California Department of Toxic Substances Control, 69 pp, 2018

This paper summarizes the primary types of ISTR, discusses their effectiveness in reducing chlorinated VOC contamination in bedrock, and provides several specific examples of full-scale implementation.

Performance Monitoring for Thermal Processes: In Situ

The three most common in situ heating technologies are steam injection (SI), electrical resistive heating (ERH), and thermal conductive heating (TCH). The table below summarizes the performance parameter(s), typical monitoring frequencies, and reason for monitoring various media. Sampling frequencies and which chemical parameters to monitor should be determined during the data quality objectives process.

Steam injection and extraction (also known as steam-enhanced extraction, or SEE) involves the introduction of steam into injection wells. Air also can be injected to induce the oxidation of some contaminants. As the steam front moves across the treatment zone, it volatilizes contaminants, displaces water with dissolved-phase contaminants, and can mobilize DNAPL masses. The contaminants, water, and potential DNAPLs are captured by recovery wells and transmitted to the surface for treatment. The steam front might follow preferential pathways created by differences in soil permeabilities. This preferential flow can result in areas in the treatment zone remaining below operational temperatures (adapted from Davis 1998).

Effluent stream monitoring is required to monitor the progress of the remediation process. Vapor- and aqueous-phase samples normally are collected at regular intervals during the course of a remediation and analyzed to determine the amount of contaminants removed. Sample intervals of one hour have been used on a small-scale demonstration project, while a one-day sampling interval was used on a full-scale demonstration; however, the time required to analyze these samples by techniques such as gas chromatography limits their usefulness for process control and optimization purposes. Effluent streams, particularly the vapor effluent stream, can vary significantly over short periods of time, and these variations cannot be monitored with the 1-day or even 1-hour periods between samples that generally are used for grab samples. Flame ionization detectors have been used with some steam injection systems for real-time monitoring of the contaminants being recovered in the vapor phase (Davis 1998).

Tracking the movement of the steam injection front in the subsurface is also desirable for monitoring the progress of the process, and to aid in understanding the processes that are occurring as a result of steam injection. Temperature measurements, either at intermediate observation wells or at the producing wells, provide a direct means of tracking steam front movement (Davis 1998). These temperature instruments can be supplemented with electrical resistive tomography (LaBrecque 2001 and Ramirez 1995) or borehole radar (Gregoire 2004) to provide a better understanding of the steam movement. Additional information is available in a separate section on Steam Injection and Extraction.

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. Because it relies on moisture to pass current, the maximum heating temperature in the vadose zone is in the 100oC range. Temperatures over 100�C can be generated in the saturated zone, and these temperatures produce steam and allow steam stripping. Volatilization and steam stripping with SVE-capture are the predominant removal mechanisms for most contaminants using this technology (Beyke and. Fleming 2002). Additional information is available in a separate section on Electrical Resistance Heating.

TCH 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. The heater wells are usually operated at a temperature between 540 and 815oC, and the soil is heated by conduction. (Baker and Heron 2004). As the soil is heated, organic contaminants in the soil are vaporized and/or destroyed by mechanisms such as evaporation, steam distillation, boiling, oxidation, and pyrolysis (ITRC 2004). Conductive heating operates best in unsaturated soil; however, it does find application in saturated soil with low hydraulic conductivity. Additional information is available in a separate section on Conductive Heating.

Depending on the contaminant, the treatment zone must reach and hold a specified temperature for the ERH or TCH to be effective. Temperature is monitored by distributing thermal couples or fiber-optic distributed temperature sensing (DTS) devices throughout the target area. Contaminants in soil vapor are frequently measured at the vapor treatment influent header to evaluate system performance and to determine when to shut down the system, such as when influent concentrations drop below a predetermined level and remain there. If condensate is produced, it can be tested for contaminants and their potential byproducts to assist in mass removal calculations. Proof of cleanup is usually accomplished through analysis of randomly distributed soil cores throughout the treatment zone and contaminant concentrations in the groundwater, if present. For some chlorinated methanes and ethanes that can be degraded by heating, testing the soil for elevated chloride levels will assist in mass removal calculations. Tracers can also be used to estimate source removal.

Additional information about nonspecific performance monitoring techniques can be found in Remediation Measurement Tools.

References

Design: In Situ Thermal Remediation
U.S. Army Corps of Engineers. EM 200-1-21, 243 pp, 2014

This document provides guidance and background for the appropriate screening and selection of in situ thermal remediation technologies, including steam enhanced extraction/injection, electrical resistivity heating, and thermal conductive heating. The document is intended to help distinguish proper applications of the technology and identify important design, operational, and monitoring issues.

In-Situ Delivery of Heat by Thermal Conduction and Steam Injection for Improved DNAPL Remediation
R. Baker and G. Heron.
Proceedings of the 4th International Conf. on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA, May 24-27, 2004. Battelle, Columbus, OH.

Steam Injection for Soil and Aquifer Remediation
E. Davis.
EPA 540-S-97-505,16 pp, 1998
Contact: Eva L. Davis, davis.eva@epa.gov

Strategies for Monitoring the Performance of DNAPL Source Zone Remedies
Interstate Technology and Regulatory Council (ITRC) Dense Nonaqueous-Phase Liquids Team. DNAPLs-5, 206 pp, Aug 2004

This document presents strategies for assessing remedial technology performance and ways in which the success or failure in treating a DNAPL source zone has been measured.

Additional Resources

Application of Borehole Radar for Monitoring Steam-Enhanced Remediation of a Contaminated Site in Fractured Limestone, Maine, USA

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Electrical Resistance Tomography for Steam Injection Monitoring and Process Control

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The Role of Advanced Monitoring in Steam Stripping for In-Situ Remediation of DNAPL

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Performance Monitoring of Remediation Technologies for Soil and Groundwater Contamination: Review
Lai, K.C.K., R.Y. Surampalli, R.D. Tyagi,, I.M.C. Lo, and S. Yan
Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management 11(3):132-157(2007)

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Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study at a Quarry at the Former Loring Air Force Base, Maine
Gregoire, C., P.K. Joesten, and J.W. Lane, Jr.
U.S. Geological Survey Scientific Investigations Report 2006-5191, 35 pp, 2007