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)
Hekster, F.M., R.W.P.M. Laane, and P. de Voogt. 2003. Environmental and toxicity effects of perfluoroalkylated substances. Reviews of Environmental Contamination and Toxicology 179:99-121.
Higgins, C. and R. Luthy. 2006. Sorption of perfluorinated surfactants on sediments. Environmental Science & Technology 40(23):7251-7256.
Kaiser, M.A., B.S. Larsen, C-P.C. Kao, and R.C. Buck. 2005. Vapor pressures of perfluorooctanoic, -nonanoic, -decanoic, -undecanoic, and -dodecanoic acids. Journal of Chemical and Engineering Data 50(6):1841-1843.
Kauck, E.A. and A.R. Diesslin. 1951. Some properties of perfluorocarboxylic acids. Industrial and Engineering Chemical Research 43(10):2332-2334.
Conductive heating 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. While it is feasible to deploy all the wells in a heater/vacuum mode, the typical deployment is to place six heater-only wells in a hexagonal shape with a heater vacuum well occupying the center of each hexagon.
The wells can be installed using conventional drilling techniques or direct push. Heater wells are constructed of steel pipe with the base sealed. A resistive heating unit is lowered into the well and current is supplied. The heating element typically operates at temperatures between 540 and 815°C (Baker and Heron 2004). The steel pipe is heated by radiant energy and the soil surrounding it by thermal conductance.
The vacuum well contains the same steel pipe and heating element components as a standard heater well, but it is placed within a larger screened well to which a vacuum can be applied. Heat propagates in a cylindrical fashion from the well outward. The heating is fairly even through all dry textures of soil. The hottest soil (typically 590°C) is in the immediate vicinity of the wells, while the coolest soil is at the midpoint between wells. When the vacuum is applied to the center well, volatilized organics are pulled through the high-temperature soil, where some of the contaminants may be degraded (Baker and Heron 2004). The extracted vapors are transported to the surface for treatment.
Well spacing is chosen based on contaminant type and depth, soil moisture content, the minimum required temperature between wells, and the time desired to reach that temperature (U.S. EPA 2004). SVOCs, including high boiling components, such as PAHs or PCBs, generally need a soil temperature of 325°C for adequate desorption, while VOCs require less heat (usually 100°C) (Baker and Heron 2004). The ability to treat high-boiling contaminants at temperatures well below their boiling points is largely due to the significant increase in vapor pressures at the temperatures present and the relatively long residence time in a very hot subsurface (Biershenk et al. 2004).The temperature requirements typically lead to well placement distances of 6 to 7.5 feet for the SVOCs and 12 to 20 feet for the VOCs. As with electrical resistance heating, the closer the wells, the faster the desired temperatures are reached.
Conductive heating operates best in unsaturated soil; however, it does find application in saturated soil with low hydraulic conductivity. As the temperature around the heater wells increases, the water evaporates and a "dry" zone is created that expands outward. At the leading edge of this cylindrical zone, steam is created, which further expands the zone. In low permeability soil, any replacement water that attempts to flow into the dry zone is quickly boiled off. In soil with high hydraulic conductivities, the influx of water to replace that boiling off may be sufficient to prevent the soil from exceeding the boiling point of water, and target temperatures may not be met. If the treatment area contains saturated high-hydraulic conductivity soil, then a dewatering system should be considered, or Baker and Heron (2004) suggest using a steam system to control water influx, as well as sweeping the permeable areas. The drying of soils, especially fine-grained silt and clay, at high temperatures can result in shrinkage and cracking that will promote the removal of organics contained within them (U.S. EPA 2004).
If concentrated halogenated organics are the contaminants of concern, the system—both piping and treatment—must be designed to withstand highly corrosive conditions.
Thermal conductance systems also can consume large quantities of power. At a site in Alhambra, California, the remediation had to be carried out in phases to avoid exceeding the capacity of the local power supplier. Vendor cost estimates cited range from $100 to $250 per ton (NAVFAC 1999). TerraTherm has an exclusive license in the United States to offer this technology for remediation.
Summarizes how the ISTD technology works, explains the underlying reaction mechanisms and rates, and demonstrates that most of the remediation is due to in situ destruction reactions.
Presents an overview of how in situ thermal conductance works and how steam can be used to improve its performance, particularly in the saturated zone.
Engineering Forum Issue Paper: In Situ Treatment Technologies for Contaminated Soil, EPA 542-F-06-013, 2006.
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