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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The halogenated alkenes, 1,1-dicloroethene (1,1-DCE), 1,2-dichloroethene (1,2-DCE), trichloroethene (TCE), and tetrachloroethene (PCE) have similar industrial uses as metal degreasers, dry cleaning agents, and intermediates in the synthesis of other chemicals. They are also used in the electrical industry and as solvents for a wide variety of paints, varnishes, and other products for the home and office. Several of these compounds, such as TCE, PCE, and 1,2-dichloroethene, have formerly been used as inhalation anesthetics, either alone or in combination with other compounds. TCE and PCE have had pharmaceutical uses as antihelminthics, such as those used to control parasitic worms in mammals. 1,3-Dichloropropene (1,3-DCP) is used as a soil fumigant to alleviate crop damage from small nematode worms that attack root systems.
In general, the halogenated alkenes are lipophilic, which is the key property that makes these compounds useful as solvents and degreasers. Furthermore, their lipophilic behavior enables them to rapidly penetrate absorption barriers, such as the respiratory and gastrointestinal tracts. They penetrate the skin more slowly. Once absorbed, they are distributed by the blood to body fat and fatty tissues, such as the brain. If absorbed in sufficient amount, compounds such as TCE, PCE, and 1,2-DCE, quickly exert the effects associated with their use as inhalational anesthetics, including depression of the central nervous system, which results in hypoactivity, ataxia, and anesthesia. Metabolism of the halogenated alkenes generally occurs in the liver, and their metabolites are excreted in urine. In some instances, the metabolites may exert their own toxic effects beyond those of the parent compound. A portion of the absorbed dose of TCE, PCE, and other compounds is exhaled unchanged.
Animal studies show that there are toxic effects associated with exposure to the halogenated alkenes that include damage to target organs, such as the liver and kidney, damage to the central nervous system, and development of benign and malignant tumors. Long-term animal studies and some epidemiological surveys suggest that some of the halogenated alkenes may be human carcinogens. Seven epidemiological studies involving well characterized exposures to TCE indicate that occupational exposure to the compound is associated with excess incidences of cancer in the liver and kidney, and non-Hodgkin's lymphoma. Additional evidence from these studies suggests an excess incidence of prostate cancer and multiple myeloma. Smaller studies have shown an increased risk of death from Hodgkin's disease and cervical cancer. An elevation in cancer mortality from cancers of the esophagus and cervix has been seen in workers exposed to PCE in dry cleaning operations. There also is some evidence to suggest an excess of kidney cancers.
No information is available on the potential developmental or reproductive toxicity of 1,2-DCE or 1,3-DCP, but studies in laboratory animals indicate that 1,1-DCE, TCE, and PCE may cause developmental abnormalities in the offspring of pregnant animals exposed to these compounds. Some epidemiological studies suggest that exposure to PCE causes an increased rate of spontaneous abortion in women, and that cardiac abnormalities are more frequently seen in the babies of women exposed to TCE in pregnancy.
Ecological Toxicity
Despite their lipophilic properties, minimal evidence exists to indicate that halogenated alkenes bioaccumulate. These compounds are volatile, and their volatility together with their limited potential for bioaccumulation may suggest that these compounds do not present a substantial toxic hazard to most ecological receptors. However, studies indicate that 1,3-DCP, which is used to control soil nematodes, is highly toxic to some species of zooplankton and juvenile sheepshead minnows. 1,3-DCP is moderately toxic to other fish species, such as largemouth bass, adult sheepshead minnows, and bluegill, and to invertebrates, such as the water flea and ramshorn snail. Fish appear to be sensitive to PCE, and this compound has been shown to cause reduced hatching, abnormal heart development, hemorrhage, and scoliosis in the Japanese medaka at low concentrations. Although some acute toxicity studies have been performed in some fish species, and in some plants and algae, minimal information exists on the effects of halogenated alkenes on terrestrial ecological receptors.
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