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.
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Halogenated alkanes are alkane molecules with one or more hydrogen atoms replaced by either chlorine, fluorine, bromine, or iodine, or by a combination of those elements. For example, dibromochloromethane (bromine and chlorine), and 1,1,2-trichloro-1,2,2-trifluoroethane (chlorine and fluorine) are two of many examples of this class of compounds. Although halogenated alkanes differ in their physical chemistry and reactivity, there are some broad similarities in their toxicological effects and modes of action.
Many of the halogenated alkanes exert general anesthetic effects by depressing the functioning of the central nervous system. Some of these compounds, chloroform for example, formerly had clinical use as inhalational anesthetics; however, the use of chloroform and similar compounds as anesthetics was discontinued in part due to another common feature of halogenated alkane toxicity, the ability to cause liver damage. At sufficient levels of exposure, most of the 21 halogenated alkane compounds considered in the DNAPL web pages are capable of causing liver damage, a condition that varies from mild to severe depending on the compound concerned. In addition, some halogenated alkanes have been reported to cause damage to the kidney, thyroid, spleen, adrenal glands, and testis.
U.S. EPA's Integrated Risk Information System (IRIS) has classified 11 of the 21 compounds as probable or possible human carcinogens. Laboratory rodent studies have reported cancers of the liver, kidney, stomach, and intestine in exposed animals.
With two notable exceptions, 1,2-dibromoethane (EDB) and 1,2-dibromo-3-chloropropane (DBCP), the chlorinated alkanes do not appear to have adverse reproductive effects, and do not appear to be toxic to the developing fetus except at doses that cause maternal toxicity. EDB and DBCP are male reproductive toxins in humans and animals.
Halogenated alkanes have given both positive and negative results on standard genotoxicity assays. EDB, DBCP, 1,2-dichloroethane, and bromodichloromethane give mostly positive results in genotoxicity tests.
Ecological Toxicity
Few studies are available that describe the ecological toxicity of the halogenated alkanes. Those studies that are available usually are acute toxicity studies that use aquatic invertebrates and fish as test subjects. With the exceptions of a few highly sensitive species, these compounds generally exert slight toxicity to aquatic species. No data were found that describe the toxicity of halogenated alkanes to terrestrial species.
For Further Information
See references appended to individual compound toxicity sections.
Proceed to "Halogenated alkanes > Bromoform"
