State-of-the-Practice Review of In Situ Thermal Technologies

In situ thermal-based soil and aquifer remediation technologies (e.g., electrical resistance heating [ERH], conductive heating, and steam-based heating) have undergone rapid development and application in recent years. These thermal technologies offer the promise of more rapid and thorough treatment of nonaqueous phase liquid (NAPL) source zones; however, their field-scale application has not been well documented in the technical literature. A state-of-the-practice review of the application of these technologies was conducted in this study. Available documents from 182 applications were reviewed, which included 87 ERH, 46 steam-based heating, 26 conductive heating, and 23 other heating technology applications conducted between 1988 and 2007. Approximately 90% of the 182 applications were implemented after 1995 and about half since 2000. More specifically, this review identified the geologic settings in which these technologies were applied, chemicals treated, design parameters, operating conditions, and performance metrics. The results of this study are summarized in a table linking this information to five generalized geologic scenarios. Practitioners considering thermal technologies for their site can identify the geologic scenario that most closely resembles their site and then can quickly see which technologies have been applied in that setting, the designs employed, operating conditions, and the performance achieved.     

Removal of PCE DNAPL from Tight Clays Using In Situ Thermal Desorption

This paper presents a full‐scale thermal remediation of a brownfields site near San Francisco, California. In Situ Thermal Desorption (ISTD) was used for treatment of chlorinated solvents in a tight clay below the water table. The site had contaminants in concentrations indicating that a tetrachloroethene (PCE)‐rich DNAPL was present. A target volume of 5097 m³ of subsurface material to a depth of 6.2 m was treated for a period of 110 d of heating. Energy was delivered through 126 thermal conduction heater borings, and vapors were extracted from a combination of vertical and horizontal vacuum wells. Approximately 2540 kg of contaminants were recovered in the extracted vapors by the end of treatment. The PCE concentration in the clay was reduced from as high as 2700 mg/kg to an average concentration of 0.012 mg/kg within 110 d of heating (a reduction of >99.999%). Similar effectiveness was documented for TCE, cis‐1,2‐DCE, and vinyl chloride. A total of 2.2 million kWh of electric power was used to heat the site. Approximately 45% of this energy was used to heat the subsurface to the target temperature. Another 53% was necessary to boil approximately 41% of the groundwater within the treatment zone, creating approximately 600 pore volumes of steam by the end of the 110‐d heating and treatment period. Steam generation thus occurred within the clay. Partitioning of the contaminants into the steam and its removal comprised the dominant remedial mechanism. The steam migrated laterally toward the ISTD heaters, where it encountered a small dry region adjacent to each of the heaters, which served as a preferential pathway allowing the steam to migrate upward along the heaters to the more permeable vadose zone. There the steam was captured by a system of vertical and horizontal vacuum extraction wells. This vapor removal strategy facilitated effective thermal treatment of the tight clays located below the water table. Features of a robust design are extension of the heaters at least 1.2 m deeper than the treatment depth, and the installation of shallow horizontal vapor collection wells which allow for establishment of pneumatic control.     

Effects of In Situ Remediation Using Oxidants or Surfactants on Subsurface Organic Matter and Sorption of Trichloroethene

In situ remediation technologies have the potential to alter subsurface properties such as natural organic matter (NOM) content or character, which could affect the organic carbon‐water partitioning behavior of chlorinated organic solvents, including dense nonaqueous phase liquids (DNAPLs). Laboratory experiments were completed to determine the nature and extent of changes in the partitioning behavior of trichloroethene (TCE) caused by in situ chemical oxidation or in situ surfactant flushing. Sandy porous media were obtained from the subsurface at a site in Orlando, Florida. Experiments were run using soil slurries in zero‐headspace reactors (ZHRs) following a factorial design to study the effects of porous media properties (sand vs. loamy sand with different total organic carbon [TOC] contents), TCE concentration (DNAPL presence or absence), and remediation agent type (potassium permanganate vs. activated sodium persulfate, Dowfax 8390 vs. Tween 80). Results revealed that the fraction of organic carbon (f_oc) of porous media after treatment by oxidants or surfactants was higher or lower relative to that in the untreated media controls. Isotherm experiments were run using the treated and control media to measure the distribution coefficient (K_d) of TCE. Organic carbon‐water partitioning coefficient values (K_oc) calculated from the experimental data revealed that K_oc values for TCE in the porous media were altered via treatment using oxidants and surfactants. This alteration can affect the validity of estimates of contaminant mass remaining after remediation. Thus, potential changes in partitioning behavior should be considered to help avoid decision errors when judging the effectiveness of an in situ remediation technology.     

Assessment of Groundwater Quality Improvements and Mass Discharge Reductions at Five In Situ Electrical Resistance Heating Remediation Sites

A recent study assessing the state‐of‐the‐practice of in situ thermal remediation technologies (e.g., electrical resistive heating [ERH], conductive heating, steam‐based heating, in situ large‐diameter auger soil mixing with steam/hot air injection, and radio‐frequency heating) identified 182 applications in the 1988 to 2007 period and summarized the geologic settings in which these technologies were applied, chemicals treated, design parameters, and operating conditions. That study concluded that documentation for less than 8% of those applications contained sufficient data to assess the effect remediation had on groundwater quality. Consequently, post‐treatment data were collected at five ERH sites, with emphasis on assessing reductions in dissolved groundwater concentrations and mass discharge (mass flux) to the aquifer. For each site, dissolved groundwater concentrations and hydraulic conductivities were determined across a vertical transect oriented perpendicular to groundwater flow and at the downgradient edge of the treatment zone. Dissolved concentration and mass discharge reductions ranged from about less than 10× to 100×, with post‐treatment groundwater concentrations ranging from about 10¹ to 10⁴μg/L and mass discharges ranging from about 10^–1 to 10² kg/y. The primary factors differentiating sites with greater and lesser dissolved concentration and mass discharge reductions were the adequacy of pre‐treatment source zone delineation, the extent to which the treatment zone encompassed the source zone, and the duration of treatment at the design operating temperature. The results suggest that ERH systems are capable of reducing groundwater concentrations to 10 to 100 μg/L levels and lower in some settings, but only if the source zone is adequately delineated and fully encompassed by the treatment system, and the treatment system is operated for a sufficiently long period of time.     

Comparison of Delivery Methods for Enhanced In Situ Remediation in Clay Till

Three methods for enhanced delivery of in situ remediation amendments in low-permeability deposits have been tested at a site in Denmark: pneumatic fracturing, direct-push delivery, and hydraulic fracturing. The testing was carried out at an uncontaminated part of a farm site, previously used for storage of chlorinated solvents, underlain by basal clay till with hydraulic conductivity ranging from 7.1× 10^–11 to 3.5 × 10^–7 m/s at testing depths 2.5 to 9.5 m b.s. Fluorescent tracers fluorescein and rhodamine WT were delivered. Tests of all three delivery methods have not been carried out at a single site before, and thus, this study provides unique data for comparison of enhanced delivery methods in both the vadose and saturated zone. Results show that pneumatic fracturing with nitrogen gas and propagation pressures of 1 to 9 bar had a distribution radius of less than 2 m, and produced dense networks of tracer-filled natural fractures above the redox boundary (0 to 3 m b.s.) and widely spaced, discrete, induced, tracer-filled subhorizontal fractures at depth (>3 m b.s.). Direct-push delivery at pressures of 8 to 30 bar had a distribution radius of approximately 1 m, distributed tracer primarily in natural fractures above the redox boundary and in discrete, closely spaced (but not merging) induced fractures below the redox boundary. Hydraulic fracturing with a sand-guar mixture at pressures of 0 to 6 bar produced an elliptical, asymmetrical, bowl-shaped fracture with a physical radius of approximately 3.5 m at 3 m b.s. The geometry of hydraulic fractures attempted emplaced at 6.5 and 9.5 m b.s. is uncertain, but clearly not horizontal as desired. The direct-push delivery method is robust and efficient for enhanced delivery at the clay till site in question, which based on thorough geological characterization is deemed a geologically representative basal clay till site.     

Arrays of Unpumped Wells for Plume Migration Control by Semi-Passive In Situ Remediation

Arrays of unpumped wells can be used as discontinuous permeable walls in which each well serves both as a means to focus ground water flow into the well for treatment and as a container either for permeable reactive media which directly destroy dissolved ground water contaminants or for devices or materials which release amendments that support in situ degradation of contaminants within the aquifer downgradient of the wells. This paper addresses the use of wells for amendment delivery, recognizing the potential utility of amendments such as electron acceptors (e.g., oxygen nitrate), electron donors (primary substrates), and microbial nutrients for stimulating bioremediation, and the potential utility of oxidizers, reducers, etc., for controlled abiotic degradation. Depending on its rate and constraints, the remedial reaction may occur within the well and/or downgradient. For complete remediation of ground water passing through the well array, the total flux of amendment released must meet or exceed the total flux demand imposed by the plume. When there are constraints on the released concentration of amendment (relative to the demand), close spacing of the wells may be required. If the flux balance allows wider spacing, it is likely that limited downgradient spreading of the released amendment will then be the primary constraint on interwell spacing. Divergent flow from the wells, roughly two times the well diameter, provides the bulk of downgradient spreading and constrains maximum well spacing in the absence of significant lateral dispersion. Stronger lateral dispersion enhances the spreading of amendment, thereby increasing the lateral impact of each well, which allows for wider well spacing.     

Evaluation of Extensive Secondary Maximum Contaminant Level Exceedances Following Remediation by In Situ Chemical Oxidation

In situ chemical oxidation (ISCO) with activated persulfate is commonly used for the remediation of petroleum impacted soil and groundwater because of its proven efficiency and the perception that reaction end products are completely innocuous. While the reaction products are less hazardous compared to the contaminants being treated, they may inadvertently prolong site closure in areas that have adopted the U.S. Environmental Protection Agency (EPA) Secondary Maximum Contaminant Levels (SMCLs) as enforceable standards. This study examines the occurrence and persistence of iron, manganese, sulfate, sodium, and total dissolved solids (TDS) in groundwater following persulfate ISCO. The concentrations of these chemicals were observed remaining above their respective regulatory criteria almost 3 years following the chemical application. Background concentrations and mobilization due to the petroleum contamination and ISCO application are also evaluated. Baseline sampling revealed substantially higher iron and manganese concentrations inside the plume area compared to the upgradient and downgradient wells suggesting mobilization due to redox reactions occurring inside of the plume. Iron was not a component in the applied chemical formula, yet the iron concentration spiked by 366% in the key monitoring well during the first post-remediation monitoring event. Ionic interactions between the ISCO amendment and native soils are believed to be responsible for displacing significant quantities of iron from the soil. Sulfate, sodium, and TDS exceedances are primarily associated with decomposition products of the ISCO amendments. The iron, manganese, sulfate, sodium, and TDS concentrations are trending downward over time, but still exceed regulatory criteria or pre-ISCO concentrations.     

The Application of In Situ Air Sparging as an Innovative Soils and Ground Water Remediation Technology

Vapor extraction (soil venting) has been demonstrated to be a successful and cost-effective remediation technology for removing VOCs from the vadose (unsaturated) zone. However, in many cases, seasonal water table fluctuations, drawdown associated with pump-and-treat remediation techniques, and spills involving dense, non-aqueous phase liquids (DNAPLS) create contaminated soil below the water table. Vapor extraction alone is not considered to be an optimal remediation technology to address this type of contamination.
An innovative approach to saturated zone remediation is the use of sparging (injection) wells to inject a hydrocarbon-free gaseous medium (typically air) into the saturated zone below the areas of contamination. The contaminants dissolved in the ground water and sorbed onto soil particles partition into the advective air phase, effectively simulating an in situ air-stripping system. The stripped contaminants are transported in the gas phase to the vadose zone, within the radius of influence of a vapor extraction and vapor treatment system.
In situ air sparging is a complex multifluid phase process, which has been applied successfully in Europe since the mid-1980s. To date, site-specific pilot tests have been used to design air-sparging systems. Research is currently underway to develop better engineering design methodologies for the process. Major design parameters to be considered include contaminant type, gas injection pressures and flow rates, site geology, bubble size, injection interval (areal and vertical) and the equipment specifications. Correct design and operation of this technology has been demonstrated to achieve ground water cleanup of VOC contamination to low part-per-billion levels.     

Using Phase Diagrams to Predict the Performance of Cosolvent Floods for NAPL Remediation

Cosolvent flooding using water miscible solvents such as alcohols has been proposed as an in situ NAPL remediation technique. This process is conceptually similar to enhanced oil recovery (EOR) using alcohols and some surfactant formulations. As a result of interest in the EOR aspects of these systems, analytical and graphical methods based on fractional flow theory were developed in the petroleum engineering literature for modeling theses floods. The existing fractional flow solutions have not been used previously in environmental applications of cosolvent flooding, but they are applicable and provide many useful insights into the process. These applications are discussed, with an emphasis on emplaining the mechanisms which tend to mobilize trapped NAPL during a cosolvent flood. The theory provides a simple way to predict the general behavior of a cosolvent flood using the phase diagram. It is concluded that the one-dimensional performance of a cosolvent flood can be predicted largely by inspection of the ternary phase diagram. In particular, the nature of the cosolvent flood depends primarily on the position of the cosolvent injection concentration relative to a critical the line extension which passes through the plait point, tangent to the binodal curve.     

Considerations for the Design of In Situ Vapor Extraction Systems: Radius of Influence vs. Zone of Remediation

When designing in situ soil vapor extraction systems, the number and placement of vapor extraction wells arc typically based on the radius of influence determined from some combination of pilot test data, theoretical considerations, and experience. In this report, we examine common methods used to determine the radius of influence, and through examples we illustrate how effective this remedial design approach is. Significant conclusions arc the following: (a) systems designed by radius of influence-based approaches may never achieve desired remedial goals: (b) systems designed by radius of influence-based approaches may result in longer operation times and higher total costs than a system that incorporates remedial goals and some level of fundamentally based predictive modeling: and (c) at best, the radius of influence-based approach ensures containment of contaminant vapors.     

Allocation of Environmental Remediation Funds Using Economic Risk-Cost-Benefit Analysis: A Case Study

Efficient allocation of remediation resources is a critical need throughout the nation. Economic risk-cost-benefit analysis is an important tool for meeting this need. This paper provides site engineers, geologists, and managers with a conceptual understanding of economic risk-cost-benefit analysis and shows how it can be applied, even in situations where existing data are sparse or poor in quality. An example analysis is applied to the remediation of radioactive waste at Oak Ridge National Laboratory, in which the cost-effectiveness is compared for two remediation alternatives: containment of the waste or monitoring only. A data-worth analysis is also carried out to estimate the maximum justifiable exploration budget and the cost-effectiveness of two proposed data collection programs. Results indicate that the methodology has potential in making robust remediation decisions regarding certain types of questions.