Office of Research and Development

National Risk Management Research Laboratory, Ground Water & Ecosystems Restoration Division, Ada, OK 74820

Eva Davis, PhD

US EPA – RS Kerr Environmental Research Center

Thermal Remediation

in Fractured Rock

January 2014
In Situ Thermal Remediation

Applications

• Aggressive technologies:

–Generally applied to Source 

Zones to Recover NAPL 

• Applicable to VOCs and SVOCs

• Applicable in wide variety of 

settings

–Simple and highly 

heterogeneous lithology

–Above and below water table

–Sites with surface structures

–Fractured bedrock

Expected Outcome in 

Porous Media

• Large percentage of mass 

recovered

• Orders of magnitude reductions 

in soil and groundwater 

concentrations

• Orders of magnitude reduction 

in mass flux to downgradient 

plume

• P&T or Monitored Natural 

Attenuation effective for 

downgradient plume

2
Mechanisms for Enhancing Recovery 

of VOCs by Thermal Remediation

•Increasing the temperature:

– Increases vapor pressure

• For VOCs, vapor pressure increases more 

than an order of magnitude going from 

ambient temperature to 100°C 

– Decreases viscosity of water and NAPL

• Most significant for viscous NAPLs

– Decreases adsorption

• May also increase desorption rate

3
For Best Results, Heat to the Boiling 

Point of Water

• Boiling Point is a “Moving Target”

• When NAPLs are present, co-boiling lowers boiling point 

below that of water alone

• Pulling vacuum on subsurface lowers boiling point

• Below water table, boiling point increases due to 

hydrostatic pressure of water

• Pressure increases in low permeability rock matrix due to 

heating will increase the boiling point

• Thus, higher temperatures/somewhat more energy

required for low permeability/sparsely fractured rock

4
Lab Tests on Rock Discs

• More than 98% of TCE 

& PCE mass removed in 

first 24 – 25 hours of 

heating to 105°C

• Heating duration had a 

greater effect on VOC 

removal than temperature

• Thermal remediation 

favored by high porosities 

and low fraction of organic 

carbon

5

TCE saturated siltstone discs inside convection 

oven. From Lebron et al., 2012
Challenges of Fractured Rock

• Characterization of contaminant distribution and thus 

defining the area to be treated

• Moving contaminants out of low permeability matrix

– During thermal remediation, pressure buildup in low permeability matrix 

will force contaminants to higher permeability zones

• Low permeability matrix, low fracture frequency increases 

the boiling point of water/contaminants

• Ensuring capture of vapors and groundwater

• These are also challenges in heterogeneous porous 

media

6
Thermal Technologies

• Three main technologies in use today:

– Steam Enhanced Extraction (SEE)

– Electrical Resistance Heating (ERH)

– Thermal Conductive Heating (TCH)

• Differ in the means by which energy is added 

to the subsurface

• Life cycle assessments show that thermal 

technologies use less energy than 

conventional SVE (Hiester and Schrenk, 2005)

7
Steam Enhanced Extraction

• Steam injected under 

pressure, with 

continuous multiphase 

extraction 

• Steam flows in fractures, 

heat conducted into 

matrix from fractures

• Co-air injection used to 

aid in upward vapor 

migration

8

Temperature distribution after 30 days of steam injection into a 100 μm 

fracture at 50 ft depth, using 0.8 psi/ft (simulated using a modified MarxLangenheim solution; Tom Heron et al. 1999)
Loring AFB SEE Research Project

• Limestone Quarry 

where drums of 

wastes had been 

buried

• Complex bedrock 

structures with 

several major 

fracture features

• Sparse fracture 

frequency and low 

permeability in 

target area

9
Characterization of the Loring Quarry

• Mapping of Fractures

• Coring during Well 

Installation

• Borehole Geophysics

• Acoustic Televiewer

• Methanol Extracted Rock 

Chip Sampling

• Discrete Interval 

Groundwater Sampling

• Head Measurements

10
Characterization of the Loring Quarry

(con’t)

• Discrete Interval 

Transmissivity Testing

• Interconnectivity 

Testing

• Deep Groundwater 

Sampling

– Two deep angled 

borings under target 

treatment area

11
Loring AFB SEE Research Project

Electrical Resistance Tomography to monitor heating

• ERT seemed to show expected heat flow in fractures

12

4 October 2002 7 November 2002 19 November 2002 30 November 2002

Percent Difference of Conductivity Percent Difference of Conductivity Percent Difference of Conductivity

TC-1 VEA-9 VEA-4

10.0

13.5

17.0

20.5

24.0

27.5

31.0

34.5

D

e

pth (m)

3.0

6.5

0.0 12.1 24.1

Iteration = 8 RMS = 9043.21%

38.0

Normalized L2 = 30.1

Percent Difference of Conductivity (%)

0.0 150 300 450 600

TC-1 VEA-9 VEA-4

6.5

10.0

13.5

17.0

20.5

24.0

27.5

31.0

34.5

D

e

pth (m)

3.0

0.0 12.1 24.1

Iteration = 8 RMS = 157.76%

Percent Difference of Conductivity (%)

0.0 150 300 450

38.0

TC-1 VEA-9 VEA-4

6.5

10.0

13.5

17.0

20.5

24.0

27.5

31.0

34.5

D

e

pth (m)

3.0

Normalized L2 = 98.82

38.0

12.1 24.1

RMS = 1010.71%

0.0

600

Iteration = 8

Percent Difference of Conductivity (%)

0.0 150 300 450 600

TC-1 VEA-9 VEA-4

Percent Difference of Conductivity

10.0

13.5

17.0

20.5

24.0

27.5

31.0

34.5

D

e

pth (m)

6.5

3.0

Normalized L2 = 123.30

38.0

0.0 12.1 24.1

Iteration = 8 RMS = 827.75% Normalized L2 = 3392.89

Percent Difference of Conductivity (%)

0.0 150 300 450 600
Loring AFB Temperature Profiles

Steam flow in fractures Heat conduction from 

steam in fractures into 

rock matrix

13
Loring Quarry SEE Conclusions

Loring Aqueous Concentrations

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

30-Aug

2-Sep5-Sep8-Sep

11-Sep14-Sep17-Sep20-Sep23-Sep26-Sep29-Sep

2-Oct5-Oct8-Oct

11-Oct14-Oct17-Oct20-Oct23-Oct26-Oct29-Oct

1-Nov6-Nov9-Nov

12-Nov15-Nov18-Nov21-Nov24-Nov

Date

Concentration (ug/L)

DRO (ug/L)

GRO (ug/L)

Solvents (ug/L)

Maximum Concentrations:

DRO (ug/L): 17600

Solvents (ug/L): 9540

• Steam injection could not 

adequately heat the entire 

target area due to sparse 

fractures and low fracture 

permeability; ERH or TCH 

more applicable for matrix 

heating

• All contaminant mass 

within 2 feet of fractures 

(steam in fractures put 

heat where it was needed)

• Significantly enhanced the 

recovery of solvents and 

gasoline- and diesel-range 

organics in the vapor and 

aqueous phases 14
SEE Applications in Fractured Rock

• Highly fractured rock with significant fracture 

permeability

• Where diffusion of contaminants into rock matrix is 

limited

• Characterization would include interconnectivity testing

• The less frequent the fractures, the more 

characterization needed to design injection/extraction 

layout

• Rock sampling also needed to determine 

injection/extraction strategy

15
Electrical Resistance Heating

8 9

10

6

1

2

3

5

4

Effluent Treatment

System

Condenser/Moisture

Separator

Vapor Phase GAC

or Thermox

Liquid Phase GAC

Vapor Collection

Manifold

Line Power

Power Control Unit

Power to Electrodes

Electrode/Vapor

Extraction Well

Computer controls

power to electrodes

& collects subsurface

temperature data

8

7

9

10

6

1

2

3

4

5

7
17

Electrical Resistance Heating for 

Remediation

• Full scale application uses 3 phase current

•Alternating current applied to electrodes in 

subsurface 

• Current carried between electrodes by water 

in pore spaces

•Electrical resistance of soils produces heat

17
18

Electrode/Vent

Electrode/Vent

Electrode/Vent

Electrode/Vent

Electrode/Vent

How Does Electrical Heating Work ?

• Triangular/Hexagonal 

Array

• A typical array diameter 

is 20 – 40 feet

• Typically 100 – 600 

volts applied per 

electrode

• Steam temperatures 

reached in ~ 3 – 6 

months

• Electrodes also typically 

vapor extraction wells

Electrode/Vent

18
19

Electrical Heating of Soils/Rock

• Heating dependent on electrical conductivity of 

water in pore space: low permeability zones 

often heated first due to higher cation content

•Works above and below water table

• Temperature limited to boiling point of water

• Contaminants collected as vapors

•Where groundwater flow rate is high, 

groundwater also extracted, can extract NAPL

• Challenged by flow rates greater than 1 ft/day

19
ERH in 

Fractured Rock

• Very hard iron-cemented 

Sandstone 40 – 65 ft bgs

• 1,1,2,2-Tetrachloroethane; 

hydrolysis formed TCE 

• Rock heats at a rate 

similar to soils

• Kluger and Beyke, 2010

20

ERH Treatment Interval

Relative CVOC Conc.

3/6/06

2/27/06

Start

TMP 4 Depth vs Temperature

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90 100 110

Temperature (°C)

Depth (ft bgs)

10

0

Depth (ft bgs)

20

30

40

50

60

70

1 10 100 1,000 10,000 100,000

Total CVOCs (μg/kg) – Log Scale

99.9% average reduction Sandstone Dense Sand

Pre-ERH CVOCs

Post-ERH CVOCs

Treatment Goal

(95% reduction)
Thermal Conductive Heating

• Heater wells with 

temperature of ~ 700°C 

installed vertically in 

triangular pattern, 12 – 20 ft 

spacing

• Co-located vapor extraction 

wells

• Heat is conducted from the 

well into the soil, dependent 

on soil thermal conductivity

21
22

Thermal Conductive Heating

• Relies on thermal conductivity of soil to heat ground

– Thermal conductivity of soils/rocks fairly uniform

– But low, ~10-6 m2/s 

• Requires high temperature (~700°C or greater) at point of 

application, close well spacing (10 – 12 ft)

• Triangular/hexagonal arrays

• Vapor extraction wells co-located with heater wells

• Treatment temperatures of 300°C possible above the 

water table

– Can treat PCBs, PAHs

22
•Rock heats generally at same rate as soils 23

Temperature - °F
TCH in Fractured Rock Research

at NAWC Site

• Heat migration not as 

adversely affected by 

geologic heterogeneity

• Rock matrix heated, high 

groundwater flow in 

fractures limited treatment 

at fracture surfaces

• Co-steam injection 

recommended for sites 

with significant fracture 

permeability

24
NAWC Temperatures during Heating 

Groundwater flow in fractures at 40-50 ft bgs limited heating

25
NAWC Rock Sampling

• Most of the mass is 

adjacent to fractures

• Post-treatment, samples 

not adjacent to fractures 

were nondetect for TCE

• TCE at fractures surfaces 

reduced from average of 

37 mg/kg to 5 mg/kg

26

26
Match Thermal Technology

to Fractured Rock Setting

• Low Permeability Matrix

– Favors ERH or TCH

• Fracture Frequency

– High frequency favors SEE

– Low frequency favors ERH or TCH

•Permeability of fractures

– Large groundwater flow through fractures favors SEE

• High Permeability Fractures/Low Permeability 

Matrix

– Combine SEE with ERH or TCH

27
Questions??

• davis.eva@epa.gov

•(580) 436-8548

28
References

• Hiester, U. and V. Schrenk. 2005. “In Situ Thermal Remediation: 

Ecological and Economic Advantages of the TUBA and THERIS 

Methods.” Proceedings of ConSoil 2005. Bordeaux, France.

• Kluger, M., and G. L. Beyke, Electrical Resistance Heating of Volatile 

Organic Compounds in Sedimentary Rock, Remediation, Autumn 

2010, pg. 69-82.

• Lebron, C. A., D. Phelan, G. Heron, J. LaChance, S. Nielsen, B. 

Kueper, D. Rodriguez, A. Wemp, D. Baston, P. Lacombe, and F. H 

Chapelle, Dense Non Aqueous Phase Liquid (DNAPL) Removal from 

Fractured Rock Using Thermal Conductive Heating (TCH), Naval 

Facilities Engineering Command Contract Report CR-NAVFAC ESCEv-1202, August 2012.

29