Valence state: The combining capacity of an atom or radical determined by the number of electrons that it will lose, add, or share when it reacts with other atoms.
free product: A NAPL found in the subsurface in sufficient quantity that it can be partially recovered by pumping or gravity drain.
aerobic: Direct aerobic metabolism involves microbial reactions that require oxygen to go forward. The bacteria uses a carbon substrate as the electron donor and oxygen as the electron acceptor. Degradation of contaminants that are susceptible to aerobic degradation but not anaerobic often ceases in the vicinity of the source zone because of oxygen depletion. This can sometimes be reversed by adding oxygen in the form of air (air sparging, bioventing), ozone, or slow oxygen release compound (e.g., ORC(r)).
Aerobic dechlorination may also occur via cometabolism where the dechlorination is incidental to the metabolic activities of the organisms. In this case, contaminants are degraded by microbial enzymes that are metabolizing other organic substrates. Cometabolic dechlorination does not appear to produce energy for the organism. At pilot- or full-scale treatment, cometabolic and direct dechlorination may be indistinguishable, and both processes may contribute to contaminant removal. For aerobic cometabolism to occur there must be sufficient oxygen and a suitable substrate which allows the microbe to produce the appropriate enzyme. These conditions may be present naturally but often in the presence of a source area oxygen and a substrate such as methane or propane will need to be introduced.
Adapted from US. EPA 2006 Engineering Issue: In Situ and Ex Situ Biodegradation Technologies for Remediation of Contaminated Sites
anaerobic: Direct anaerobic metabolism involves microbial reactions occurring in the absence of oxygen and encompasses many processes, including fermentation, methanogenesis, reductive dechlorination, sulfate-reducing activities, and denitrification. Depending on the contaminant of concern, a subset of these activities may be cultivated. In anaerobic metabolism, nitrate, sulfate, carbon dioxide, oxidized metals, or organic compounds may replace oxygen as the electron acceptor.
Anaerobic dechlorination also may occur via cometabolism where the dechlorination is incidental to the metabolic activities of the organisms. In this case, contaminants are degraded by microbial enzymes that are metabolizing other organic substrates. Cometabolic dechlorination does not appear to produce energy for the organism. At pilot- or full-scale treatment, cometabolic and direct dechlorination may be indistinguishable, and both processes may contribute to contaminant removal.
Quoted from US. EPA 2006 Engineering Issue: In Situ and Ex Situ Biodegradation Technologies for Remediation of Contaminated Sites
architecture: "Architecture" refers to the physical distribution of the contaminant in the subsurface. Residuals that take the form of long thin ganglia or small dispersed globules provide a larger surface area that will dissolve much faster than if the same amount of liquid were concentrated in a competent pool.
Sources: For purposes of this discussion, a DNAPL source zone includes the zone that encompasses the entire subsurface volume in which DNAPL is present either at residual saturation or as "pools" of accumulation above confining units. In addition, the DNAPL source zone includes regions that have come into contact with DNAPL that may be storing contaminant mass as a result of diffusion of DNAPL into the soil or rock matrix.
source zone: For purposes of this discussion, a DNAPL source zone includes the zone that encompasses the entire subsurface volume in which DNAPL is present either at residual saturation or as "pools" of accumulation above confining units. In addition, the DNAPL source zone includes regions that have come into contact with DNAPL that may be storing contaminant mass as a result of diffusion of DNAPL into the soil or rock matrix.
focal ulceration: The process or fact of a localized area being eroded away.
metaplasia of the glandular stomach: A change of cells to a form that does not normally occur in the tissue in which it is found.
hyperplasia of the glandular stomach: A condition in which there is an increase in the number of normal cells in a tissue or organ.
histiocytic: Degenerative.
duodenum: First part of the small intestine.
microcytic: Any abnormally small cell.
squamous cell papillomas: A small solid benign tumor with a clear-cut border that projects above the surrounding tissue.
squamous cell carcinomas: Cancer that begins in squamous cells-thin, flat cells that look under the microscope like fish scales. Squamous cells are found in the tissue that forms the surface of the skin, the lining of hollow organs of the body, and the passages of the respiratory and digestive tracts. Squamous cell carcinomas may arise in any of these tissues.
jejunum: The middle portion of the small intestine, between duodenum and ileum. It represents about 2/5 of the remaining portion of the small intestine below duodenum.
ileum: The distal and narrowest portion of the small intestine.
squamous: Flat cells that look like fish scales.
metaplasia: A condition in which there is a change of one adult cell type to another similar adult cell type.
ossification: The process of creating bone, that is of transforming cartilage (or fibrous tissue) into bone.
clastogenesis: Any process resulting in the breakage of chromosomes.
neoplastic: Abnormal and uncontrolled growth of cells.
ulceration: The process or fact of being eroded away.
leucocytosis: An elevation of the total number of white cells in blood.
neutrophils: A type of white blood cell.
chromodulin: A small protein that binds four trivalent chromium ions.
biomagnification: The increased accumulation and concentration of a contaminant at higher levels of the food chain; organisms higher on the food chain will have larger amounts of contaminants than those lower on the food chain, because the contaminants are not eliminated or broken down into other chemicals within the organisms.
exencephaly: Cerebral tissue herniation through a congenital or acquired defect in the skull.
everted viscera: Rotated body organs in the chest cavity.
To Be Considered: Documents, such as federal or state guidances, that are not legally binding but may be relevant to the topic in question.
gaining: A gaining surface water body is one where groundwater flows into it.
losing: A surface water body is losing when there is a permeable sediment bed that is not in contact with the groundwater allowing the surface water to seep through it.
fluvial: Of or pertaining to flow in rivers and streams.
lacustrine: Of or pertaining to a lake as in lacustrine sediments—sediments at the bottom of a lake.
lipid: Any class of fats that are insoluble in water.
lipophilic: Able to dissolve in lipids—in this case fatty tissue.
organelles: A part of a cell such as mitochondrion, vacuole, or chloroplast that plays a specific role in how the cell functions and membranes.
RfD: The RfD is an estimate of a daily exposure of the human population (including sensitive sub-groups) to a substance that is likely to be without "the appreciable risk of deleterious effects during a lifetime." An RfD is expressed in units of mg/kg-day.
autonomic: That part of the nervous system that controls non-conscious actions such as heart rate, perspiration and digestion.
ataxia: Lack of muscle coordination.
funnel-and-gate configuration: A system where low-permeability walls (the funnel) placed in the saturated zone direct contaminated ground-water toward a permeable treatment zone (the gate)
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Electrical resistance heating (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. Originally, ERH for remediation purposes was developed by DOE and Battelle using six-phase electricity and hexagonal electrode arrays; however, expansion of a six-phase hexagonal array may result in the creation of cold and hot spots within the target subsurface area. Three-phase power and triangular electrode arrays generally are more applicable to irregularly shaped cleanup areas for full-scale treatment and are commonly used today (Beyke and Fleming 2005).
Electrodes can be thought of as wells that are equipped to deliver electric power at selected depths and also act as vapor recovery wells. When groundwater flow rates are high and/or a semivolatile NAPL is to be recovered, liquids can also be extracted (Beyke and Fleming 2002). Electrodes may be installed using conventional drilling rigs. Installation can also be done using horizontal or angular drilling techniques. Care should be taken to ensure that the potential for stray currents is accounted for in the design. ERH systems can be deployed to any depth and used in both the vadose and saturated zone. If the system is deployed only in the vadose zone, water should be added at the electrodes to maintain the moisture content and thus the flow of electricity (U.S. EPA 2004).
The horizontal spacing between electrodes is usually between 14 and 24 feet (Beyke and Fleming 2005). The trade-off in distance is between the cost of installing more electrodes and heating the soil more quickly or installing fewer electrodes and heating the soil over a longer time.
While heating all soil, ERH preferentially heats the more conductive silt and clay first. Temperatures over 100°C can be generated in the saturated zone, and these temperatures produce steam and steam stripping, which is especially beneficial for the silts and clays as contaminant movement in them is usually diffusion limited.
Volatilization and steam stripping with SVE-capture are the predominant removal mechanisms for most contaminants using this technology. (Beyke and Fleming 2005). Soil with a high natural organic carbon content will slow or prevent the recovery of some organic contaminants.
U.S. EPA (2004) provides remediation cost estimates of $32 and $73 per cubic yard at two full-scale ERH sites. Beyke and Fleming (2005) estimate that ERH costs $200,000, plus $40 to $70 per cubic yard. The technology is proven and has been used at a number of sites. The number of vendors offering the technology is limited.
Information on applications of electrical resistance heating technology specific to a chemical class can be found in the class subsections listed to the right.
Discusses the full scale remediation of Area A of the Northeast Site at the Young-Rainey STAR Center, Largo, FL. The site was contaminated with NAPL constituents such as TCE, cis-1,2-DCE, methylene chloride, toluene, and petroleum hydrocarbons. The hydrogeology consists of a fine-grained sand aquifer underlain by Hawthorne clay at 30 feet depth. The upper 5 feet of this clay formed part of the remediation volume, as DNAPL was present in this layer. Steam enhanced extraction and electrical resistance heating were used to remediate the site.
Provides a case study of the application of steam heating and ERH to a mixture of DNAPL chemicals and LNAPL oils at the Young-Rainey Science, Technology, and Research (STAR) Center, Largo, FL.
Engineering Forum Issue Paper: In Situ Treatment Technologies for Contaminated Soil, EPA 542-F-06-013, 2006.
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