ATV Jord og Grundvand

Afværgeteknologier – State of the Art

Schæffergården, Gentofte 22. oktober 2008

HEATING TECHNIQUES - STATUS, IMPORTANT PARAMETERS - EXPERIENCES, OPTIMIZATION AND

DEVELOPMENT POSSIBILITIES

Gorm Heron, Vice President, ph.d. TerraTherm Inc.

Incl. attachments:

HEAT IT ALL THE WAY

- MECHANISMS AND RESULTS ACHIEVED USING IN-SITU THERMAL REMEDIATION

USE OF THERMAL CONDUCTION HEATING FOR THE REMEDIATION

OF DNAPL IN FRACTURED BEDROCK

Gorm Heron, Vice President, ph.d. Ralph S. Baker, Chief Executive Officer

John M. Bierschenk, President John C. LaChance, Senior Project Manager

TerraTherm Inc.

SUMMARY In-situ thermal remediation has evolved significantly in the last 5 years, and it’s use is becom-ing more and more wide-spread. To date, approximately 180 sites in North America and ap-proximately 20 sites in the rest of the world have been treated using one or more of the domi-nant thermal technologies:

• Electrical Resistance Heating, also called Electro-Thermal Dynamic Stripping Proc-ess,

• Steam Enhanced Extraction, also called Steam Injection, and • In-Situ Thermal Desorption, also called Thermal Conductive Heating.

Each technology has it’s own niche and sweet-spot applications. The principle of the energy delivery is shown in Figure 1. The main mechanisms are thermal conduction for ISTD, resis-tive/ohmic heating for ERH, and steam injection and flow for SEE. All of the techniques are applicable for VOC contaminants located above the groundwater table. Only ISTD is effective for SVOC contaminants that need treatment at temperatures above 120oC. ERH and ISTD is challenged by high groundwater flow as cooling occurs, whe-reas SEE is applicable in such zones where the steam is readily injected /1/. Trends observed in the last decade include:

• Thermal methods have been used to achieve better than 99% mass reduction, and most often is used only in source zones.

• In the US, many thermal projects are guaranteed to meet the desired results. The con-fidence in the technologies is growing.

• ERH is the most frequently used thermal technology. There are 3 vendors in the US; ISTD is represented by one, and SEE is offered by 2 vendors.

• ISTD generally is used for sites with the most stringent goals (target soil concentra-tions below 1 mg/kg).

• SEE is used for deep and large sites without thick clay layers. The unit cost of SEE is lower for such sites. There have been few such applications, but some large projects.

• For VOC treatment of relatively large clays sites, both ERH and ISTD has been effec-tive and reached unit treatment costs in the range of $100-200 per cubic meter. Small sites have much higher unit costs.

• Combinations of technologies have been used with great success only by a few ven-dors. When SEE is combined with either ERH or ISTD, treatment of both clay layers and high-flowing aquifers can be done simultaneously.

• ISTD and SEE have or is being been used in Denmark at a total of approximately 7 si-tes. ERH has been used at a few sites in The Netherland and Belgium.

In general, ISTD and ERH apply to the same VOC sites, except for those where only ISTD has been effective:

• Sites with very stringent remedial goals

• Sites with contaminants with high boiling points, requiring heating to above 120oC • Fractured rock sites. An example ISTD application in saprolite and gneiss is provided

in an attached paper /2/ An accompanying paper presents the thermodynamic basis for thermal treatment, and some guidelines for implementation /1/. In the presentation, an overview of the heating methods, and a summary of results achieved, will be provided.

ComparisonThermal Conduction Heating (TCH), or In-Situ Thermal

Desorption* (ISTD)

Electrical Resistance Heating(ERH) – Joule Heating

Steam Enhanced Extraction(SEE) – Steam Injection

Figure 1. Illustration of the three major thermal technologies and how the energy is delivered. Brown layers rep-resent clays, the gray layer represents a permeable sand zone. Note that ERH for deep sites requires stacked elec-trodes, whereas ISTD uses a single heater pipe. Steam can flow readily in the sand, but not in the clays

REFERENCES ATTACHED

/1/ Heron, G., R.S. Baker, J.M. Bierschenk and J.C. LaChance. 2006. “Heat it All the Way - Mechanisms and Results Achieved using In-Situ Thermal Remediation.” Remediation of Chlorinated and Recalci-trant Compounds: Proceedings of the Fifth International Conference (May 22-25, 2006). Battelle, Co-lumbus, OH.

/2/ Heron, G., R.S. Baker, J.M. Bierschenk and J.C. LaChance. 2008. “Use of Thermal Conduction Heat-ing for the Remediation of DNAPL in Fractured Bedrock.” Remediation of Chlorinated and Recalci-trant Compounds: Proceedings of the Fifth International Conference (May 19-22, 2008). Battelle, Co-lumbus, OH

HEAT IT ALL THE WAY - MECHANISMS AND RESULTS ACHIEVED USING IN-SITU THERMAL REMEDIATION Gorm Heron, TerraTherm, Inc., Keene, CA, USA) Ralph S. Baker, John M. Bierschenk, and John C. LaChance, TerraTherm, Inc., Fitchburg, MA, USA) ABSTRACT In situ thermal remediation technologies have been proven to reach very low soil and ground-water concentrations by eliminating the dense non-aqueous phase liquid (DNAPL) source and reducing dissolved and adsorbed chlorinated volatile organic compound (CVOC) concentra-tions to near non-detect levels. For chlorinated solvents, vaporization is the dominant mecha-nism, as vapor pressure and Henry’s law constants increase most markedly with temperature. For effective treatment, pneumatic and hydraulic control must be achieved during the heating period, and a clear path for the generated vapors to an extraction system must be provided. If remedial goals are stringent, target temperatures shall be the in-situ boiling point of the soil and groundwater system, such that a phase change to the vapor state is forced by the heating. During operation, detailed temperature monitoring and process sampling is conducted and compared to the performance calculated based on mass and energy balances. Interim and final sediment sampling is used to verify remedial progress and performance prior to site demobili-zation. INTRODUCTION Recently, results from sites that were heated and treated using in-situ thermal remediation (ISTR) have indicated impressive removal rates for DNAPL source zones in soil and ground-water. Published results from both a U.S. Department of Energy (DOE) site (Young-Rainey STAR Center; Heron et al. 2005), and an industrial facility in the Midwest (LaChance et al. 2004) have documented mass removal efficiencies in the 99.9% range. Soil concentrations of Contaminants of Concern (COCs) below or near non-detect are reported, and groundwater concentrations near or below Maximum Concentration Limits (MCLs) have been observed inside the original source zones. These results appear almost unrealistic, considering the re-calcitrant nature of DNAPLs in the subsurface, long-term diffusion processes, heterogeneity of most source zones, and frequently raised questions about DNAPL capture at thermal sites. Other site reports, particularly from sites where Electrical Resistance Heating (ERH) was used, have reported less impressive results, sometimes less than 90% mass removal (Lowry Landfill, CO: Plaehn et al. 2004; ICN Pharmaceuticals, Portland, OR: USEPA, 2004; Navy Bedford, MA: Francis and Wolf, 2004). As more data are emerging, it is becoming evident that thermal remediation spans a wide range of heating methods, and that applications vary from very robust, effective systems to poorly designed and ineffective systems. This paper reviews the mechanisms behind ISTR critically, focusing on what happens at the pore and micro-scale, as well as larger scale during heating. It will review ways for the con-taminants to be mobilized and extracted, and show the most proper design for vapor recovery and capture systems. Several key design elements will be presented, in an attempt to

explain why not all thermal projects have achieved the success made possible by the theory, and how achievement of desired results can be much more widely attained. THERMAL REMEDIATION METHODS ISTR is gaining acceptance for restoration of NAPL source zones (Davis, 1997). The follow-ing ISTR methods are discussed below:

• Steam Enhanced Extraction (SEE). • Electrical Resistance Heating (ERH). • Dynamic Underground Stripping (DUS). • In-Situ Thermal Desorption (ISTD).

Steam has been used to heat the more permeable zones, which are typically sandy layers with relatively low clay and mineral contents. The in-situ process using steam injection and ag-gressive fluids extraction was named Steam Enhanced Extraction (Udell et al. 1991), and sev-eral field demonstrations and full-scale cleanups have been conducted (Udell et al. 1999; Eaker 2003; EarthTech and SteamTech 2003). Mechanisms used in SEE were reviewed criti-cally (Udell 1996). Both three-phase and six-phase ERH were developed as robust techniques in the 1990’s and demonstrated in the field. ERH involves passing electricity through the soil between elec-trodes, and heating the soil by Joule heating. Laboratory studies demonstrated that thermody-namic changes induced by ERH can lead to very effective removal of chlorinated solvents from silts and clays (Heron et al. 1998). Since the late 90’s, several commercial full-scale im-plementations of both three- and six-phase ERH were completed, some by the trade name Electro-Thermal Dynamic Stripping Process (ET-DSP) (McGee 2003). The combination of steam and ERH is named Dynamic Underground Stripping, and was demonstrated at a gasoline spill that had resulted in LNAPL contamination above and below a rising water table at the Livermore Gas Pad (Newmark 1994; Daily et al. 1995). This method was used recently to remediate a DNAPL source area at the Young-Rainey STAR Center (He-ron et al. 2005). A robust method for heating soils and groundwater is thermal conduction heating, also named In-Situ Thermal Desorption (ISTD; Stegemeier and Vinegar 2001). ISTD is a soil remediation technology in which heat and vacuum are applied simultaneously. Heat flows into the soil primarily by conduction from heaters typically operated between 500 and 800°C, with the tar-get soil volume being heated to 100°C for VOC removal. As the soil is heated, water is boiled and DNAPL constituents in the soil are vaporized. The resulting steam and vapors are drawn toward extraction wells for in-situ and aboveground treatment. Compared to fluid injection processes, the conductive heating process is very uniform in its vertical and horizontal sweep.

Other thermal methods such as hot water flooding, hot air sparging, and radio-frequency heat-ing were not consider during this review. This review focuses on volatile compounds, and therefore on the methods for which heating to the boiling point of water is sufficient. The higher temperature version of TerraTherm’s ISTD technology, used for treatment of semi-volatile organic compounds (SVOCs) such as polychlorinated biphenyls (PCB) and coal tar, will not be discussed. REMEDIATION MECHANISMS FOR CVOC CONTAMINANTS For thermal treatment of VOC DNAPL, the dominant mechanism is vaporization, as illus-trated in Figure 1, showing how boiling leads to steam formation and gas flow rich in con-taminant vapors out of the pore matrix. Note the continuous gas phase in the right image whe-re pore fluids are boiling and creating steam, which sweeps out to recovery wells. Boiling oc-curs at DNAPL-water interfaces and throughout.

1 mm

Figure 1. Conceptual illustration of the difference between ambient temperature (left) and boiling temperature conditions (right) at the pore scale

Figure 2 summarizes the physical property changes occurring during heating for water, tri-chloroethene (TCE), and tetrachloroethene (PCE). While DNAPL density, viscosity, surface tension, and solubility varies slightly, vapor pressure and Henry’s law constants increase dra-matically with temperature.

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Other mechanisms include enhanced dissolution, hydrolysis, and aqueous phase oxidation. However, vaporization is dominant for most chlorinated solvents. HOW CLEAN CAN IT GET? Table 1 shows the results achieved using DUS combined with detailed flexible monitoring, sampling, energy balance calculations, and careful pneumatic and hydraulic control. Removal efficacies of 99.9% or better were achieved.

Table 1. Treatment efficiency based on mass estimates from soil sampling before and after DUS treatment at the Young-Rainey STAR Center, Largo, FL (Heron et al. 2005).

Number of sam-

ples

TCE (µg/kg)

cis-1,2-Di- chloroethene

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Methylene Chloride (µg/kg)

Toluene

(µg/kg)

Before, maxi-mum

250,000 68,000 650,000 72,000

Before, average 231

2,753 1,239 3,444 825 After, maximum 110 120 8.2 420 After, average

80 3.3 4.4 0.8 21

Removal efficiency (%) 99.99 99.85 99.99 99.89

Table 2 shows similar contaminant mass reduction achieved using ISTD to heat a tight satu-rated clay to 100°C and vaporizing 40% of the groundwater.

Table 2. Comparison of COC concentrations before and after ISTD treatment at the Terminal 1 site in Rich-mond, CA (Geomatrix and TerraTherm 2005).

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ples

PCE (µg/kg)

TCE (µg/kg)

cis-1,2-Di-chloroethene

(µg/kg)

Vinyl chlo-ride

(µg/kg)

Before, maxi-mum

510,000 6,500 57,000 6,500

Before, average 64

34,222 1,055 6,650 932 After, maximum 44 < rep.limit 1,500 24 After, average

44 12 < rep.limit 65 4.7

Removal efficiency (%) 99.96 99.63 99.03 99.49

These results illustrate the dramatic reduction in source zone CVOC concentrations when the technology is used to reach boiling point temperatures and operated with a sufficient energy input to vaporize more than 30% of the soil moisture. CRITICAL DESIGN AND IMPLEMENTATION ISSUES Select the Most Appropriate Heating Technology Often site owners or consultants fail to consider all the options for thermal treatment and rely on one concept and cost when making a decision. It is important to realize, for instance, that ERH and ISTD often apply to the same sites, and that sometimes one method is more appro-priate. Key issues for the selection includes treatment size (ERH often is cheapest for small sites with modest remedial goals), treatment depth (ISTD is simpler and more cost-effective for deep sites), treatment goals (ISTD is often used for sites with very stringent cleanup goals), and site permeability (SEE may be more applicable to deep permeable formations). Therefore, clients and consultants should not restrict themselves to working with a single thermal vendor, but ought to request evaluations from more than one for each site. Establish Hydraulic and Pneumatic Control Before heating, inward flow of fluids (vapor and water) must be ensured. During SEE and DUS, inward flow of groundwater and vapor must also be ensured. Detailed calculation of necessary rates must be performed (steam can rapidly displace large quantities of fluids). Dur-ing ERH and ISTD, vapor capture must be ensured by a robust vacuum extraction system that allows a flow pathway for the steam and CVOC vapors to extraction points. One cannot rely on steam bubbles migrating upward to the vadose zone, or on clay drying to create permeabil-ity – the generated vapor must flow readily to extraction wells screened in the right zones. During ERH and ISTD, hydraulic control must be maintained, either by boiling sufficient amounts of groundwater to create capture, pumping, or through use of a hydraulic barrier. Heat to Target Temperature High Enough to Accomplish the Remedial Objectives For most CVOCs, except those which degrade readily by hydrolysis such as Methylene Chlo-

ride (MeCl2), 1,2-Dichloroethane (1,2-DCA), and 1,1,1-Trichloroethane (1,1,1-TCA), the tar-get treatment temperature should be the boiling point of the pore water or groundwater. Heat-ing to 100oC or slightly lower where vacuum is applied ensures that all DNAPL is vaporized and removed, and that steam stripping will reduce dissolved and adsorbed concentrations (Udell, 1996). One mistake is to rely on the psychrometric effect (air mixing with steam and lowering the boiling point), since typical heterogeneity makes it impossible to distinguish whether a zone heated to 85oC is being cleaned due to air flow or whether it is stagnant and by-passed by the heating (Heron et al. 2005). Ultimately this reduces the certainty of reaching cleanup goals in a timely manner. Below the water table the target temperature increases with depth due to the increased pres-sure. At a depth of 10 m below the water table, the groundwater boils at approximately 120oC. Target temperatures must be the boiling temperature to ensure steam stripping. During operation, subsurface temperature monitoring is essential (e.g. Heron et al. 2005). For heterogeneous sites, thermocouples should be placed no more than 1.5 m apart vertically, and a network of monitoring wells should cover the target area and the zones around it. Mass and Energy Balance Calculations and Data Management A key to a good thermal project design and execution is careful energy management and monitoring of the progress in different ways – an example of the use of a detailed energy bal-ance for finding problems is presented in Figure 3. At this site, the energy balance indicated that a cool zone was present (average site temperature was 80-90oC, but the thermocouples showed 100-110oC), and subsequent drilling revealed a recalcitrant zone, which was then tar-geted for more intense treatment. The results were encouraging, showing the importance of complete heating, and the value of careful monitoring and engineering checks. Use Pressure Cycling during Steam Enhanced Extraction During SEE and DUS, steam breaks through to dedicated extraction wells (Udell et al. 1991). The principle of pressure cycling is illustrated in Figure 4. The steam stripping is enhanced during the depressurization step, where the pressure in the steam zone is reduced. This leaves the pore fluids in the steam zone and the surrounding condensation zones at slightly super-heated conditions since the equilibrium temperature is lower than the actual temperature (see the inserted steam pressure curves at the top of the figure). The zones respond by releasing energy to get to a lower equilibrium temperature – this happens by boiling pore fluids. The generated steam and BTEX-vapors migrate in the steam zone towards recovery wells. This was documented to lead to large increases in vapor-phase recovery during full-scale remedia-tion (Heron et al 2005). Failure to conduct pressure cycling will prolong the heating period and reduce treatment efficiency.

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Figure 3. Energy balance for the steam and ERH project at Young-Rainey STAR Center Area A. The blue curve is calculated average treatment zone temperature (Heron et al. 2005).

Figure 4. Illustration of pressure cycling during steam injection and extraction. The left figure illustrates the pressurization and heating stage, the right the situation during de-pressurization.

Use Process Stream Sampling to Document Mass Removal Rates over Time

A flexible approach to sampling and data collection can not only enhance the operation of the thermal system, it can shorten the time to reach remedial goals and lower overall costs. Ex-traction wells and manifolds are hot and under vacuum – no standard EPA methods exist for collection and sample analysis. However, screening-level sampling can reveal areas where problematic compounds persist and reveal area where treatment is complete (diminishing re-turns). Examples are published by US DOE (2003). Use Interim Soil Sampling to Document Remedial Progress Interim drilling during thermal remediation can be performed safely when not drilling into significant steam zones (Gaberell et al. 2002). The data can be used to document remedial progress and final performance before operation is completed. It is useful for gathering infor-mation in zones where contaminant extraction continues despite a sense that heating is com-plete, as shown by Heron et al. (2005). Soil and sediment sampling is particularly useful since potential rebound after cooling would be the result if partitioning of contaminants from the sediments to groundwater. Since the sampling represents the potential source for rebound, it is preferred over waiting for cooling and then performing groundwater sampling. CONCLUSIONS This review revealed how effective thermal remediation can be for CVOC sites if designed and implemented well. It identified these critical design elements:

• CVOC sites with stringent treatment goals must be heated to the boiling point of wa-ter, and a significant amount of energy spent on boiling groundwater.

• Pneumatic and hydraulic control must be maintained, and a path for the generated steam to extraction wells must be included.

• Pressure cycling is important on steam projects – it shortens remediation time and in-creases effectiveness.

• Mass and energy balance calculations are useful for overall process verification, trou-bleshooting, and assurance that site heterogeneities are addressed.

• Interim process and soil sampling are important for tracking remedial performance and verifying treatment prior to cessation of operations.

Remedial standards of below 1 mg/kg have been consistently met by properly engineered ISTR systems. REFERENCES Daily, W.D., A.L. Ramirez, R.L. Newmark, K.S. Udell, H.M. Buettner, and R.D. Aines. 1995. Dynamic Under-ground Stripping: Steam and electric heating for in situ decontamination of soils and groundwater. US Patent # 5,449,251.

Earth Tech and SteamTech. 2003. Environmental Restoration Program, Site 61 treatability study report, Steam injection, Northwest Main Base, Operable Unit 8, Edwards Air Force Base, California, September.

Eaker, Craig 2003. Southern California Edison Company, Visalia Pole Yard, Visalia, California, Draft paper in preparation for submission.

Davis, E.L. 1997. How heat can accelerate in-situ soil and aquifer remediation: important chemical properties and guidance on choosing the appropriate technique. US EPA Issue paper EPA/540/S-97/502.

Francis, J. and J. Wolf. 2004. “In Situ Remediation of Chlorinated VOCs and BTEX Using Electrical Resistance Heating.” Paper 2B-19, in: A.R. Gavaskar and A.S.C. Chen (Eds.), Remediation of Chlorinated and Recalcitrant Compounds —2004. Battelle Press, Columbus, OH. Gaberell, M., A. Gavaskar, E. Drescher, J. Sminchak, L. Cumming, W-S. Yoon, and S. De Silva. 2002. Soil Core Characterization Strategy at DNAPL Sites Subjected to Strong Thermal or Chemical Remediation. Paper 1E-07, in: A.R. Gavaskar and A.S.C. Chen (Eds.), Remediation of Chlorinated and Recalcitrant Compounds.

Geomatrix and TerraTherm, 2005. Final report for ISTD treatment at Terminal 1, City of Richmond. Geomatrix Consultants, Oakland, CA.

Heron, G., M. van Zutphen, M.; T.H. Christensen, and C.G. Enfield. 1998. Soil heating for enhanced remedia-tion of chlorinated solvents: A laboratory study on resistive heating and vapor extraction in a silty, low-permeable soil contaminated with trichloroethylene. Environmental Science and Technology, 32 (10), 1474-1481.

Heron, G. S. Carroll, and S.G.D. Nielsen. 2005. Full-Scale Removal of DNAPL Constituents using Steam En-hanced Extraction and Electrical Resistance Heating. Ground Water Monitoring and Remediation, 25 (4), Winter 2005, pp. 92-107.

LaChance, J.C., R.S. Baker, J.P. Galligan, and J.M. Bierschenk. 2004. Application of Thermal Conductive Heat-ing/In-Situ Thermal Desorption (ISTD) to the Remediation of Chlorinated Volatile Organic Compounds in Satu-rated and Unsaturated Settings. Proceedings of Battelle’s Conference on Remediation of Chlorinated and Recal-citrant Compounds, Monterey, CA, May 24.

McGee, B.C.W. 2003. Electro-Thermal Dynamic Stripping Process for in situ remediation under an occupied apartment building. Remediation, Summer: 67-79.

Newmark, R.L. (ed.) 1994. Demonstration of Dynamic Underground Stripping at the LLNL Gasoline Spill Site. Final Report UCRL-ID-116964, Vol. 1-4. Lawrence Livermore National Laboratory, Livermore, California.

Plaehn, D.R., T. Powell, G. Beyke, S. Richtel, D. Bollmann and J. Herzog. 2004. “Remediation of a Waste Pit Using Electrical Resistance Heating.” Paper 2B-13, in: A.R. Gavaskar and A.S.C. Chen (Eds.), Remediation of Chlorinated and Recalcitrant Compounds —2004. Battelle Press, Columbus, OH. Stegemeier, G.L., and Vinegar, H.J. 2001. Thermal Conduction Heating for In-Situ Thermal Desorption of Soils. Ch. 4.6, pp. 1-37. In Chang H. Oh (ed.), Hazardous and Radioactive Waste Treatment Technol. Handbook, CRC Press, Boca Raton, FL.

Udell, K.S., N. Sitar, J.R. Hunt, and L.D. Stewart. 1991. Process for In Situ Decontamination of Subsurface Soil and Groundwater. US Patent # 5,018,576.

Udell, K.S. 1996. Heat and mass transfer in clean-up of underground toxic wastes. In Annual Reviews of Heat Transfer, Vol. 7, Chang-Lin Tien, Ed.; Begell House, Inc.: New York, Wallingford, UK, pp. 333-405.

Udell et al. 1999. Alameda Point Site 5 Steam Enhanced Extraction Demonstration. Draft Final Report submitted to US Navy. Berkeley, CA.

US DOE (2003). Pinellas Environmental Restoration Project. Northeast Site Area A NAPL Remediation Final Report. Young - Rainey STAR Center. U.S. Department of Energy, Grand Junction Office, Grand Junction, Colorado. September.

U.S. EPA. 2004. “Cost and Performance Report, Electric Resistive Heating at the ICN Pharmaceutical Site, Port-land, OR. February 2004.” In: In-Situ Thermal Treatment of Chlorinated Solvents: Fundamentals and Field Ap-plications. Office of Solid Waste and Emergency Response, Office of Superfund Remediation and Technology Innovation. EPA 542-R-04-010. March.

USE OF THERMAL CONDUCTION HEATING FOR THE REMEDIATION OF DNAPL IN FRACTURED BEDROCK Gorm Heron, TerraTherm, Inc. Keene, CA, USA Ralph S. Baker, John M. Bierschenk, and John C. LaChance, TerraTherm, Inc. Fitchburg, MA, USA ABSTRACT This paper presents the first full-scale remediation at a fractured rock site using Thermal Con-duction Heating (TCH), also known as In-Situ Thermal Desorption (ISTD). A 90-ft deep TCE source area was treated thermally, including thick zones of saprolite and gneiss bedrock. The thermal treatment used 24 heater borings/wells, and operated for 148 days, after which an av-erage temperature of approximately 100oC was achieved. The ISTD remediation work was highly successful at reducing soil, rock and groundwater concentrations at this confidential facility. Post remediation soil sampling indicated that the 95% UCL of the mean concentra-tion of TCE in soil within the treated area was 17 μg/kg. This was significantly lower than the remedial goal of 60 μg/kg. In addition, groundwater concentrations within the treatment zone were reduced by between 74.5% and 99.7%. The total mass of VOCs removed from the subsurface during the ISTD remediation was approximately 12,000 lbs, almost all of which was TCE. INTRODUCTION Prior to this project, there did not exist an effective technology for the remediation of DNAPL in fractured bedrock systems. This is because DNAPL in fractured bedrock presents several significant challenges; including: 1) defining the area to be treated; 2) potential impacts of matrix diffusion within and downgradient of the source zone; 3) discrete nature of fracture pathways and presence of dead-ends; and 4) accessing DNAPL within the fractures and the contaminant mass in the matrix. One technology however, that may be able to overcome many of these limitations is Thermal Conduction Heating also known as In-Situ Thermal De-sorption. ISTD is the simultaneous application of heat, by TCH, and vacuum to the subsurface to remove organic chemicals. Heat is applied by installing electrically powered heaters at regular intervals throughout the zone to be treated. The heat moves out into the inter-well re-gions primarily by thermal conduction. Thermal conduction heating of fractured bedrock sites is capable of: 1) achieving thorough heating of the bedrock (matrix and fractures), 2) preventing unwanted condensation of steam and CVOC vapors, and 3) capture and removal of the CVOC mass liberated from the bedrock and unconsolidated deposits. In fractured rock settings, a substantial fraction of the contaminant mass may be located in the rock matrix, dissolved in matrix porewater, adsorbed to mineral surfaces and organic matter, or even as tiny droplets or ganglia if the DNAPL has entered the matrix. As a result, it is not sufficient to only apply a remedy to the fracture systems, since back-diffusion and transport of contaminants back out of the matrix can make it impossible to achieve satisfactory plume concentration reductions. Therefore, an effective fractured rock remedy must involve treat-ment of the contaminants in the matrix.

For thermal treatment of VOC DNAPL and dissolved and adsorbed phases, the dominant re-moval mechanism is vaporization, as illustrated in Figure 1 for an equivalent porous medium showing how boiling leads to steam formation and gas flow rich in contaminant vapors out of the pore matrix. Note the continuous gas phase in the right image where pore fluids are boil-ing and creating steam, which sweeps out to recovery wells. Boiling occurs at DNAPL-water interfaces and throughout. This mechanism has led to very effective thermal treatment even of thick saturated clay layers (Geomatrix and TerraTherm 2005, LaChance et al. 2004).

1 mm

Figure 1. Conceptual illustration of the difference between ambient temperature (left) and boiling temperature conditions (right) at the pore scale for a porous medium. Figure 2 summarizes the physical property changes occurring during heating for water, tri-chloroethene (TCE), and tetrachloroethene (PCE). While DNAPL density, viscosity, surface tension, and solubility varies slightly, vapor pressure and Henry’s law constants increase dra-matically with temperature (Heron et al 2006).

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Figure 2. Properties of water, PCE and TCE as a function of temperature (from Heron et al. 2006). Other mechanisms include enhanced dissolution, hydrolysis, and aqueous phase oxidation. However, vaporization is dominant for most chlorinated solvents. In fractured rock systems, boiling of fluids in the fractures and the matrix leads to steam for-mation. The steam will sweep out of the rock towards locations with low pressure. Therefore, vacuum extraction is applied to each heater boring, creating a path for the generated vapors out of the formation. By using each heater boring for extraction, it is ensured that the pro-duced steam can be extracted, and not migrate in unwanted directions. This principle is simi-lar to the one developed for ISTD treatment of tight clay zones (LaChance et al 2006). SITE DESCRIPTION AND TCH DESIGN At a site located in the southeastern part of the U.S., TCH was used to remediate a TCE DNAPL source zone that extended 90 ft below the ground surface (bgs). The bottom 15 feet of the treatment zone consisted of fractured gneiss (TerraTherm, 2007). In summary, the Site was underlain by 4 geologic units:

• Fill: The fill was 25 feet thick, had a hydraulic conductivity of 1 x 10-4 cm/sec, a porosity of 42 percent and a soil moisture content of 10.8 percent by weight.

• Saprolite: The saprolitic soil (severely weathered granitic gneiss) was 30 feet

thick and had a hydraulic conductivity, porosity, and soil moisture content of 5 x 10-5 cm/sec, 40 percent, and 14 percent, respectively.

• Partially Weathered Bedrock: Partially weathered rock (PWR) was present imme-diately beneath the saprolitic soil. It also had a hydraulic conductivity and poros-ity of 5 x 10-5 cm/sec and 40 percent, respectively. This layer was 20 feet thick.

• Fractured Bedrock: In general, the bedrock was assumed to be fractured, with a hydraulic conductivity of 1 x 10-5 cm/sec and a fracture porosity of 0.5%. The bedrock surface undulated with a typical depth to the bedrock surface of between 75 and 80 ft. There was a possibility of a highly fractured zone beneath the target treatment zone (TTZ) oriented north-to-south. The hydraulic conductivity of the highly fractured zone was assumed to range between 1 x 10-4 and 1 x 10-3 cm/s and the porosity was assumed to be 2%.

The water table at the Site was at the bottom of the saprolitic soil at approximately 55 ft bgs, resulting in a total saturated thickness of approximately 25 feet of soil and partially weathered bedrock overlying the fractured bedrock. The primary contaminant of concern (COC) present in the subsurface) at the Site that the ISTD system was designed to treat was TCE. The TCE at the site was apparently released via a sump/catch basin system associated with an aboveground TCE storage tank and a TCE rec-lamation unit (Tank Area). The amount of TCE released to the subsurface was unknown. The source area, i.e. the TTZ that the ISTD system was designed to treat was located adjacent to the southwest corner of the existing manufacturing building. The TTZ was selected in or-der to encompass the highest soil concentrations and the most likely locations of TCE present as DNAPL and included an area approximately 33 ft wide by 76 ft long (2,554 ft2) with the long axis oriented north-to-south. This alignment also coincided with the axis of the highest groundwater concentrations. The design basis for the bottom of the TTZ was 87 feet below ground surface (bgs) to encompass variations in the top of the bedrock and to ensure that all fill, saprolite, and weathered bedrock within the horizontal limits of the TTZ were treated. The heated interval extended to ap-proximately 90 ft bgs to ensure uniform heating of the bottom of the TTZ. The average depth to the top of bedrock observed based on the installation of the heater-only and heater-vacuum wells was approximately 79 ft. Thus all of the fill, saprolite, and weathered bedrock within the horizontal limits of the TTZ were treated. This amounted to approximately 8,230 cubic yards (cy) of soil and weathered bedrock. The primary remedial action objective for the ISTD installation was to remove TCE and other CVOCs present from the unsaturated and saturated portions of the TTZ (i.e., above and below the water table within the TTZ) and to attain remedial standards. Although final remedial standards were not established site-wide, the ISTD design was based on the achievement of 60 μg/kg of TCE for soil in the unsaturated zone. Although similar treatment levels for soil beneath the water table were feasible, the possibility that TCE present below the water table

outside of the TTZ could migrate back into the TTZ following treatment necessitated that no specific remedial standard be set for the saturated portion of the TTZ. Instead, the ISTD sys-tem was designed to operate until the 60 μg/kg remedial standard for unsaturated soil was be-lieved to have been achieved based on measurements of temperature and concentrations of CVOCs in the well field vapor stream and interim soil and groundwater data. At that point, the ISTD system was to be shut down and the soil and groundwater present in the saturated portion of the TTZ would be sampled and monitored to determine the level of cleanup achie-ved below the water table. Numerical simulations of the application of ISTD at the Site were performed prior to ISTD system design to provide a basis for development of the conceptual design. As a result of the numerical simulations, a target treatment temperature of 212°F (100oC) achieved in the inter-well regions and the removal of a small fraction (i.e., 20%) of the water from the TTZ was found to be sufficient to achieve the remedial standards for TCE. Because the other CVOCs present in the TTZ had similar physical and chemical properties (e.g., boiling points) as TCE, they were also found to be effectively removed from the TTZ by achieving 212°F (100oC) in the interwell regions. Thus, a target treatment temperature of 212°F (100oC) was selected for the project. A secondary remedial action objective of the ISTD installation at the Site was to minimize the potential for contaminant mobilization during treatment. Given the information available for the site at the time of the ISTD design, high concentrations of TCE and DNAPL were thought to be present in the subsurface. Thus, the ISTD system was designed to minimize the poten-tial for contaminant mobilization outside of the TTZ both vertically and laterally. Specific aspects of the design that were added to minimize the potential for contaminant and DNAPL mobilization included: Hot Floor Extension of the heaters into the upper approximately 10 to 15 ft feet of the bedrock and boosting the power output of the bottom portion of the heaters in order to establish a “hot floor.” The objective of the hot floor was to provide a barrier to vertical migration of the con-taminants as the contaminants would be volatized and extracted from the subsurface when coming into proximity with the hot floor.1 As described above, this resulted in the heating and treatment of the upper 15 to 20 ft of the fractured bedrock. Establishment of Upward Vertical Gradients A low-flow extraction system was designed to slightly lower the groundwater table within the TTZ, thereby creating upward hydraulic gradients across the bottom of the TTZ. The creation of upward gradients across the bottom of the TTZ was designed to offset the downward forces acting on the DNAPL and to provide an added level of security to ensure that the DNAPL did not migrate downward.

1 TerraTherm holds exclusive license to several patents for implementation of a hot floor during remediation to prevent verti-cal mobilization (U.S. Patent No. 5,997,214 and international patents granted and pending).

Perimeter Heater-Vacuum Well Because there was a potential that COCs existed up to the edge of the TTZ, heater-vacuum wells were placed around the perimeter of the TTZ to ensure that vapors were pulled back to-wards the TTZ and not pushed outward. The well-field is shown in Figure 3. Figure 4 shows the completed system, with fiberglass pipe manifold and a concrete vapor cover. A total of 24 heater wells/borings were used, ten of which were also used for vapor extraction.

Figure 3. Heater boring/well locations and thermocouple locations at the Site. Heater wells are open circles. Heater wells with applied vacuum and circles with a red center. Thermocouples are denoted by a red “T” within a circle.

Figure 4. Completed ISTD system.

Electrical power (1,500 kW) for the ISTD remediation project was supplied from the existing plant building electrical service. TerraTherm’s electrical distribution panels and all down-stream equipment, which was configured for 480V, 3-phase, 4-wire service, was wired to the secondary side of the transformer provided by the host facility. The vapor collection system at the Site consisted of a moisture knock-out pot and a vacuum blower to draw the heated vapors from the ground and convey them to the existing stack via a fiberglass manifold piping system that was constructed by TerraTherm. The liquid conden-sate from the manifold piping system and the liquid extracted from the recovery wells was piped to the existing groundwater treatment system at the host facility. RESULTS ISTD operations ran continuously 24 hours per day, 7 days per week from the start of heating on January 29, 2007 through the end of the heating period on June 20, 2007 and the final ISTD system shutdown on June 25, 2007. Thermocouples (TCs) were installed at 7 locations between the thermal wells throughout the ISTD well field to monitor the soil heat up. These TCs were used to determine when the tar-get treatment temperature was attained within and at the top and bottom of the TTZ. Attain-ment of the target treatment temperature was used to gauge when the ISTD treatment could be stopped. Each temperature monitoring location consisted of an array of thermocouples lo-cated at selected vertical intervals (e.g., 18, 40, 68, and 83 feet bgs). This vertical array of thermocouples enabled evaluation of the ISTD system treatment pro-gress in the various geological layers found at the Site.

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Figure 5 shows the thermocouple temperature readings at the 83 feet bgs depth, which is in the bedrock. Similar or higher temperatures were achieved at the shallower depths as well. The target temperature of the boiling point of water was generally achieved in the entire treatment volume after approximately 100 days of heating. Since the mass removal continued to be measureable and the power usage was lower than expected, the client chose to extend the operational period by approximately 7 weeks after the initial heat-up. Figure 6 shows the cumulative mass removal curve along with the projected and actual energy usage.

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The total mass of VOCs extracted from the subsurface in the vapor phase during the course of the project was approximately 11,590 pounds (5.8 tons). The total mass of VOCs extracted in dissolved liquid phase was approximately 92 pounds. In addition to the VOCs extracted through volatilization and the dissolved phase, it is expected that some additional mass of VOCs would have been eliminated in situ due to hydrolysis or other in-situ degradation proc-esses such as direct oxidation or pyrolysis. The total amount of energy used to reach the remedial goal after 110 days of heating was 1,500,000 kWh. The amount of electrical energy expended per volume treated, was 182 kWh per cubic yard. The total operating time and amount of energy that was estimated to be re-quired to heat up the TTZ and attain the remedial goal was 120 days and 2,600,000 kWh, re-spectively. Thus, the amount of energy actually used to heat up the TTZ was 60% less than the design. This indicates that subsurface heat losses to areas surrounding the TTZ were lower

than anticipated, and the applied energy was used efficiently to raise the temperature inside the TTZ. This is great news for thermal remediation in fractured rock. After 110 days of heating, a total of 66 discrete soil samples were collected and of these, 10 were duplicates. All of the duplicates showed agreement indicating very little sample vari-ability. Soil and rock samples from within the treatment zone show a very thorough removal of contaminants. Measured starting concentrations of TCE were as high as 81,000,000 μg/kg and 1,100,000 μg/L in soil and water, respectively, and DNAPL was visually observed in soil and water samples. The post-remediation 95% Upper Confidence Limit (UCL) of the mean TCE soil concentration for the entire treatment zone, above and below the water table (based on 56 discrete soil samples), was 17 μg/kg (TerraTherm, 2007). The post-treatment concen-tration of TCE in groundwater samples from a monitoring well within the treatment zone that had starting TCE concentrations at saturation levels (1,100,000 μg/L) was reduced to <5 μg/L. CONCLUSIONS In summary, the ISTD remediation work was highly successful at reducing soil and ground-water concentrations at the facility. Post remediation soil sampling indicated that the 95% UCL of the mean concentration of TCE in soil within the treated area was 17 μg/kg. This was significantly lower than the remedial goal of 60 μg/kg. In addition, groundwater concentra-tions within the treatment zone were reduced by between 74.5% and 99.7%. The total mass of VOCs removed from the subsurface during the ISTD remediation was approximately 12,000 lbs, almost all of which was TCE. This project demonstrated that ISTD can be very effective for heating and treating fractured bedrock, and that concentrations can be reduced from DNAPL-levels to near non-detect in the rock and groundwater. The mobilized and vaporized DNAPL constituents can be safely ex-tracted and treated using vacuum extraction wells, ensuring capture. This is very promising news for fractured rock sites, where conventional wisdom has been that DNAPL problems are too complex and difficult to solve with available remediation techniques. REFERENCES Geomatrix and TerraTherm. 2005. Final Report for ISTD Treatment at Terminal 1, City of Richmond, Califor-nia. Geomatrix Consultants, Oakland, CA.

Heron, G., R.S. Baker, J.M. Bierschenk and J.C. LaChance. 2006. “Heat it All the Way - Mechanisms and Re-sults Achieved using In-Situ Thermal Remediation.” Paper F-13, in: Bruce M. Sass (Conference Chair), Reme-diation of Chlorinated and Recalcitrant Compounds—2006. Proceedings of the Fifth International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2006). ISBN 1-57477-157-4, published by Battelle Press, Columbus, OH, www.battelle.org/bookstore.

LaChance, J.C., R.S. Baker, J.P. Galligan, and J.M. Bierschenk. 2004. “Application of ‘Thermal Conductive Heating/In-Situ Thermal Desorption (ISTD)’ to the Remediation of Chlorinated Volatile Organic Compounds in Saturated and Unsaturated Settings.” Paper 2B-21, in: A.R. Gavaskar and A.S.C. Chen (Eds.), Remediation of Chlorinated and Recalcitrant Compounds—2004. Proceedings of the Fourth International Conference on Reme-diation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2004). ISBN 1-57477-145-0, pub-lished by Battelle Press, Columbus, OH, www.battelle.org/bookstore

LaChance, J., G. Heron and R. Baker. 2006. “Verification of an Improved Approach for Implementing In-Situ Thermal Desorption for the Remediation of Chlorinated Solvents.” Paper F-32. , in: Bruce M. Sass (Conference Chair), Remediation of Chlorinated and Recalcitrant Compounds—2006. Proceedings of the Fifth International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2006). ISBN 1-57477-157-4, published by Battelle Press, Columbus, OH, www.battelle.org/bookstore.

TerraTherm, Inc. 2007. Remedial Action Completion Report. Implementing In-Situ Thermal Desorption (ISTD) Remediation. [Confidential SE US Site]. November.