B
Glossary

A

Abiotic

Refers to chemical transformations that occur without the aid of microorganisms.

Adsorption

The adherence of ions or molecules in solution to the surface of solids.

Advection

The process whereby solutes are transported by the bulk mass of flowing fluid.

Aerobic biodegradation

The degradation of compounds by microorganisms in the presence of oxygen. In aerobic biodegradation, microorganisms convert oxygen to water in the process of transforming other compounds to simpler products.

Air sparging

The injection of air below the water table to strip volatile contaminants from the saturated zone and to promote contaminant biodegradation.

Aliphatic hydrocarbons

A class of compounds built from carbon and hydrogen joined in open chains.

Alkane

A chemical composed of a straight chain of carbon atoms bonded on all sides by hydrogen atoms and containing no double bonds between carbon atoms. The simplest alkane is methane.

Alkene

A chemical composed of a straight chain of carbon and hydrogen atoms containing at least one double bond between carbon atoms. The simplest alkene is ethene (also known as ethylene).

Amphiphilic

A compound, such as a surfactant, that has an end with



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Alternatives for Ground Water Cleanup B Glossary A Abiotic Refers to chemical transformations that occur without the aid of microorganisms. Adsorption The adherence of ions or molecules in solution to the surface of solids. Advection The process whereby solutes are transported by the bulk mass of flowing fluid. Aerobic biodegradation The degradation of compounds by microorganisms in the presence of oxygen. In aerobic biodegradation, microorganisms convert oxygen to water in the process of transforming other compounds to simpler products. Air sparging The injection of air below the water table to strip volatile contaminants from the saturated zone and to promote contaminant biodegradation. Aliphatic hydrocarbons A class of compounds built from carbon and hydrogen joined in open chains. Alkane A chemical composed of a straight chain of carbon atoms bonded on all sides by hydrogen atoms and containing no double bonds between carbon atoms. The simplest alkane is methane. Alkene A chemical composed of a straight chain of carbon and hydrogen atoms containing at least one double bond between carbon atoms. The simplest alkene is ethene (also known as ethylene). Amphiphilic A compound, such as a surfactant, that has an end with

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Alternatives for Ground Water Cleanup an affinity for water and an end with an affinity for substances that lack an affinity for water. Anaerobic biodegradation The degradation of compounds by microorganisms in the absence of oxygen. Analytical model A model that can be solved using classical mathematical tools, such as differential equations. Anoxic Describes an environment without oxygen. Aquifer An underground geologic unit that stores ground water. Aquitard A bed of subsurface materials that retards but does not prevent the flow of water from an upper aquifer to a lower aquifer. Area of attainment The area in which cleanup standards are to be achieved. Aromatic hydrocarbon A chemical formed from benzene rings, originally called "aromatic" because of its distinctive aroma. B Bedding plane A plane that separates layers in the subsurface. Bioavailability The availability of a compound for biodegradation, influenced by the compound's location relative to microorganisms and its ability to dissolve in water. Biodegradation The biologically mediated conversion of a compound to simpler products. Bioremediation Exploiting the metabolic activity of microorganisms to transform or destroy contaminants. Biotransformation Chemical alteration of organic compounds brought about by microorganisms. Bioventing The process of passing air through the soil to stimulate biodegradation of organic contaminants. The goal is to maximize aerobic biodegradation while minimizing volatilization. BTEX An acronym for benzene, toluene, ethylbenzene, and xylenes, which are volatile, monocyclic aromatic compounds present in coal tar, petroleum products, and various organic chemical product formulations. Buffering capacity The capacity of water in a specific location to maintain its pH by neutralizing added acids or bases. C Capillary force A force that draws a fluid into the small interstices in the subsurface. Capillary fringe zone The zone immediately above the water table. Carbonate A rock formed primarily from carbonate minerals, such as limestone and dolomite. CERCLA See "Comprehensive Environmental Response, Compensation, and Liability Act of 1980."

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Alternatives for Ground Water Cleanup Chemotaxis The movement of bacteria toward a chemical that they may use as an energy source. Chlorinated solvent A solvent containing at least one chlorine atom in its chemical structure. Typically, these compounds are used to dissolve substances that do not dissolve easily in water. Because they are used for a wide variety of purposes—from manufacturing, to degreasing, to dry cleaning—chlorinated solvents are common ground water contaminants. Colloid A particle that has a diameter in the range of 10-8 to 10-5 meters. The small size of colloids tends to keep them in suspension for long time periods. Cometabolism The simultaneous metabolism of two compounds, in which the degradation of the second compound (the secondary substrate) depends on the presence of the first compound (the primary substrate). For example, in the process of degrading methane, some bacteria can degrade hazardous chlorinated solvents that they would otherwise be unable to attack. Complexation A reaction in which a metal ion and one or more anionic ligands chemically bond. Complexes often prevent the precipitation of metals. Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) A law that established a national program to respond to past releases of hazardous substances into the environment. CERCLA created the "Superfund" for financing remedial work not undertaken by responsible parties. Approximately 1,200 sites are scheduled for cleanup under the CERCLA program. Confined aquifer An aquifer bounded above and below by units of distinctly lower hydraulic conductivity and in which the pore water pressure is greater than atmospheric pressure. Confining bed A subsurface layer that inhibits the downward flow of water. Conservative solute A chemical that does not react with the soil or ground water or undergo biological, chemical, or radioactive decay. Containment Refers to systems that prevent the further spread of contamination. These systems control the ground water flow direction around the contaminated site by using pumps, injection wells, and cutoff walls placed at strategic locations. Conventional pump-and-treat systems Systems that extract contaminated ground water and treat it at the surface. Cosolvent A compound that enhances the water solubility of organic contaminants. Cyclic hydrocarbon A chemical composed of carbon and hydrogen atoms bonded in ring structures.

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Alternatives for Ground Water Cleanup D Darcy's Law A formula used to describe fluid flow in the subsurface. The law states that the velocity of flow through a porous medium is directly proportional to the hydraulic gradient (assuming that the flow is laminar and that inertial forces can be neglected). Dechlorination A process whereby chlorine atoms are removed from a compound. Complete dechlorination makes compounds less toxic and easier to biodegrade. Denitrification The conversion of nitrate to nitrogen gas by microorganisms. Denitrification can be an important process in the subsurface, because when oxygen is absent, denitrifying bacteria can use nitrate to degrade hazardous compounds in the same way that they would ordinarily use oxygen. Density The mass per unit volume of a substance. Desorption The release of sorbed molecules from the solid into solution (the reverse of sorption). Diffusion Contaminant movement caused by the random motion of molecules. Contaminants diffuse from areas of high concentration to areas of low concentration. Dispersion The spreading and mixing of chemical constituents in ground water. Dispersion is caused by diffusion and mixing due to microscopic variations in velocities within and between pores as well as by macroscopic velocity variations among zones of differing hydraulic conductivity. Dissolution The process by which solid or nonaqueous-phase liquid components of a contaminant dissolve in infiltration water and form a ground water contaminant plume. The duration of remediation measures (either cleanup or long-term containment) is determined by the rate of dissolution that can be achieved in the field and the mass of soluble contaminants. DNAPL An acronym for denser-than-water nonaqueous-phase liquid— an organic liquid, composed of one or more contaminants, that does not mix with water and is denser than water. The most common DNAPL contaminants in ground water are chlorinated solvents. E Electron acceptor substrate See "Substrate." Electron donor substrate See "Substrate." Enhanced oil recovery Processes (such as cosolvent or steam flooding) for recovering additional oil (or other nonaqueous-phase liquids) from the subsurface. Enzyme A protein that a living organism uses in the process of degrading a specific compound. The protein serves as a catalyst in the compound's biochemical transformation.

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Alternatives for Ground Water Cleanup Extraction well A well used to remove contaminated ground water from the subsurface. F Facultative anaerobe A type of bacteria that can function with or without oxygen. Because the oxygen supply in ground water is often limited, these bacteria can be important players in degrading subsurface contaminants. Feasibility study A study carried out at a hazardous waste site covered under the Comprehensive Environmental Response, Compensation, and Liability Act to determine possible remedies for contamination at the site. Fiber spectroscopy A remote method for determining contaminant presence or concentration in water by sending a light pulse down a light-transmitting fiber whose end is submerged in the water (for example, through a well) and interpreting a signal returned via that or another fiber. Fixation Mixing of contaminated soils with a chemical stabilizer, usually a cementatious grout compound. Fixed-film reactor A laboratory vessel used to simulate microbial activity in the subsurface. The vessel is filled with a granular material, such as sand, with films of bacteria attached to the surfaces of the material. Fraction of organic carbon A soil's organic carbon content, expressed as a mass fraction of the dry soil. Fractured medium A large subsurface rock or clay formation that is mostly solid but contains cracks that can transmit or store water. Free liquid diffusivity The diffusion coefficient for a solute in a dilute aqueous solution. Functional group A reactional component of a chemical compound. G Gas chromatograph An instrument used to identify volatilizable chemical contaminants in water. Geomorphology The features of the earth's surface. H Halogenated compound A compound in which one or more hydrogen atoms have been replaced by a halogen atom, such as fluorine, chlorine, or bromine. Examples include chlorinated solvents (such as 1,1,1-trichloroethane, trichloroethylene, and tetrachloroethylene), which have been widely used in cleaning and degreasing operations in some fumigant pesticides. Many halogenated compounds are DNAPLs. Henry's Law constant The equilibrium ratio of the partial pressure of a compound in air to the concentration of the compound in water at a

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Alternatives for Ground Water Cleanup reference temperature. It is sometimes referred to as the air-water partition coefficient. Heterogeneity Refers to nonuniformity in properties of a subsurface porous medium. Homogeneity Refers to subsurface media that are relatively uniform. Horizontal well A well placed horizontally in the subsurface, through which water may be injected or extracted or air may be injected to aid the volatilization of contaminants. Humic substance A macromolecular organic substance formed from the decomposition of plant or animal material. Hydraulic barrier A barrier to flow caused by system hydraulics, such as a line of ground water discharge caused by extraction wells. Hydraulic conductivity A measure of the volume of water at the existing kinematic viscosity that will move in a unit time under a unit hydraulic gradient through a unit area of medium measured at right angles to the direction of flow. Hydraulic containment See "Containment." Hydraulic gradient The change in head per unit distance in a given direction, typically in the principal flow direction. Hydrolysis A chemical reaction that involves splitting one chemical bond and adding the elements of water. Hydrophilic "Water loving"; refers to compounds that are highly water soluble. Hydrophobic "Water fearing"; refers to substances that are relatively insoluble in water. I Igneous rock A rock that solidified from molten material. "Igneous" is one of the three categories (igneous, metamorphic, and sedimentary) into which all rocks are divided. Immunoassay A method used to detect whether contaminants are present based on their ability to bind to antibodies produced by a living organism in response to the contaminant. In situ In place, i.e., within the contaminated aquifer itself. In situ vitrification The heating of the subsurface to extremely high temperatures to destroy organic contaminants. Upon cooling, the subsurface solidifies, incorporating inorganic contaminants and ash. Infiltration gallery A trench, basin, or sprinkler system used to add nutrients and energy sources to the subsurface to stimulate the growth of bacteria that can degrade hazardous compounds. Injection well A well through which fluids (water or air) are injected into the subsurface. Interfacial tension A measure of the forces at the interface between two fluids.

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Alternatives for Ground Water Cleanup Interphase mass transfer The net transfer of chemical compounds between two or more phases. Interstitial velocity A measure of the speed at which water travels through pores and other openings underground. More precisely, it is the rate of discharge of ground water per unit area of the geologic medium per percentage volume of the medium occupied by voids measured at right angles to the direction of flow. Intrinsic permeability A measure of the relative ease with which a porous medium can transmit a liquid under a potential gradient. Intrinsic permeability is a property of the medium and is dependent on the shape and size of the openings through which the liquid moves. Ion A molecule that has a positive or negative electric charge. Ion exchange The exchange of ions between a solution and a solid while maintaining charge balance. Through ion exchange, charged molecules that are naturally part of the subsurface soil may be replaced by contaminant molecules. Isotherm An equation that relates the mass of contaminant sorbed to a solid to the concentration of the contaminant in solution at equilibrium. Isotope Refers to the fact that a chemical element in the periodic table may have two or more species that behave nearly identically chemically but have different atomic masses and physical properties. K Kinetic Refers to the speed of a process. Kriging A statistical procedure that geologists use to characterize the subsurface; kriging maximizes the information obtained from a given number of samples. L Leachate The liquid that leaks from a contaminant source as water percolates through the source area and leaches chemicals from the waste. Lens A geologic deposit bounded by converging surfaces, in a shape similar to a convex lens. Lenticular Layered. Lithology A description of the rocks beneath the ground at a site. LNAPL An acronym for less-dense-than-water nonaqueous-phase liquid. LNAPLs do not mix well with water and are less dense than water. Gasoline and fuel oil are common LNAPLs. M Mass balance An attempt to determine the fate of a compound in a unit of space by evaluating all possible sources and sinks of the compound in the space, applying the principle of mass conservation. Mass spectrometer An instrument used to identify the chemical composition of a fluid.

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Alternatives for Ground Water Cleanup Mass transfer The transfer of contaminant mass from one medium to another. Mass transfer from an undissolved contaminant pool to water occurs through dissolution; mass transfer from a contaminant pool or water to air occurs through volatilization. Maximum contaminant level (MCL) The maximum amount of a compound allowed in drinking water under the Safe Drinking Water Act. MCLs are set by considering both health effects of the compound and technical feasibility of removing the compound from the water supply. Maximum contaminant levels goal (MCLG) Nonenforceable health goals established under the Safe Drinking Water Act intended to protect against known and anticipated adverse human health effects with an adequate margin of safety. Technical feasibility is not considered in setting MCLGs. Metabolism The chemical reactions in living cells that convert food sources to energy and new cell mass. Metamorphic rock A rock created from preexisting rocks in response to changes in temperature, pressure, shearing stress, or chemical environment. Methanogenesis The production of methane by bacteria. Because they thrive without oxygen, methanogenic bacteria can be important players in the subsurface, where oxygen is often absent. Methanotrophic bacteria Bacteria that grow using methane as their primary energy source. Methanotrophic bacteria may be important in cleaning up ground water, because in the process of degrading methane, they may also metabolize hazardous contaminants that are ordinarily resistant to biodegradation. Micelie A colloidal aggregate of amphiphilic molecules. The interior of micelles is hydrophobic and can dissolve hydrophobic contaminants. Mineralization The complete conversion of an organic compound to inorganic products. Miscibility The ability of two liquids, such as alcohol and water, to mix without splitting into separate phases. Model An attempt to represent processes, such as water and contaminant flow, by mathematical equations. Monitoring well A tube or pipe, open to the atmosphere at the top and to water at the bottom, used for taking ground water samples. Monomer A molecule that can combine with like molecules to form a polymer. N NAPL An acronym for nonaqueous-phase liquid, a liquid consisting of organic compounds that are not completely miscible with water (see also ''DNAPL'' and "LNAPL"). National Oil and Hazardous Substances Pollution Contingency Plan

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Alternatives for Ground Water Cleanup (NCP) The regulation describing how the mandates from Congress specified in the Comprehensive Environmental Response, Compensation, and Liability Act will be carried out in practice. Numerical model A model whose solution must be approximated by varying the values of controlling parameters and using computers to solve approximate forms of the model's governing equations. O Octanol-water partition coefficient A measure that indicates the extent to which a compound is attracted to an organic phase (for which octanol is a proxy) and hence the compound's tendency to sorb to subsurface materials. It is computed by dividing the amount that will dissolve in octanol by the amount that will dissolve in water. The greater the value, the greater the tendency to sorb in the subsurface. Olefin Synonymous with "alkene." Organic carbon partition coefficient A measure that indicates the extent to which a compound will sorb to the solid organic content of geologic media in the subsurface. It is computed as the ratio of the amount of chemical sorbed per unit weight of organic carbon in the soil to the concentration of the chemical in solution at equilibrium. Oxic Describes an environment that contains oxygen. Oxidation reaction The transfer of electrons away from a compound to another compound. Oxidation reactions are important in the destruction of contaminants. They may occur spontaneously when the appropriate chemicals are mixed, or they may be catalyzed by microorganisms. For example, when microbes degrade organic compounds, they may transfer electrons away from the compound, converting the compound to carbon dioxide and deriving energy from the electron transfer process. P Partition coefficient A term used to describe the relative amount of a substance partitioned between two different phases, such as a solid and a liquid. It is the ratio of the chemical's concentration in one phase to its concentration in the other phase. Partitioning A chemical equilibrium condition in which a chemical's concentration is apportioned between two different phases according to the partition coefficient. Permeability A measure of the relative ease with which fluids will flow through subsurface materials. Pleistocene A geologic time period associated with the Ice Age. Plume A zone containing predominantly dissolved contaminants and sorbed contaminants in equilibrium with the dissolved contaminants. A plume usually will originate from the contaminant source areas

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Alternatives for Ground Water Cleanup and extend downgradient for some distance, depending on site hydrogeologic and chemical conditions. Polar molecule A molecule that has a center of positive charge and a center of negative charge; polar compounds are usually hydrophilic. Polychlorinated biphenyl (PCB) A type of contaminant built from two benzene rings and chlorine atoms. PCBs are very stable, resisting both chemical and biological degradation, and are toxic to many species. At one time, they were used commonly in electrical transformers as heat insulators. Polycyclic aromatic hydrocarbon (PAH) A compound built from two or more benzene rings. Sources of PAHs include fossil fuels and incomplete combustion of organic matter (in auto engines, incinerators, and even forest fires). Pore A small pace between the grains of sand, soil, or rock in the subsurface. Ground water is stored and transmitted in pores. Pore volume The total volume in the pores in a given section of the subsurface. This term often is used to describe the volume of water that is contained within a contaminant plume. Porosity The volume of open spaces in the subsurface. Porous medium A subsurface zone composed of small rocks or sand particles with pores that can transmit or store water. Primary substrate A substance that can supply microorganisms with energy for growth and reproduction. Pulsed pumping An enhancement to a pump-and-treat system in which extraction wells are periodically not pumped. This method theoretically allows contaminant concentrations in the readily extracted ground water to increase by contaminant mass transfer from various subsurface zones in which the contaminants are retained. R Rate-limiting step The step in a process that limits the speed at which the process can occur. RCRA See "Resource Conservation and Recovery Act." Recharge The replenishment of water beneath the earth's surface, usually through percolation through soils or connection to surface water bodies. Record of Decision A document issued by the Environmental Protection Agency specifying the remedy for contamination at a site covered under the Comprehensive Environmental Response, Compensation, and Liability Act. Redox potential Describes the distribution of oxidized and reduced species in a solution at equilibrium. Redox potential is important for predicting the likelihood that metals will precipitate from ground water upon pumping, for estimating the capacity of microorganisms

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Alternatives for Ground Water Cleanup to degrade contaminants, and for predicting other subsurface reactions. Reduction reaction The transfer of electrons to a compound from another compound (see also "oxidation reaction"). Oxidation-reduction reactions are important in the destruction of contaminants. They may occur spontaneously, when the appropriate chemicals are combined, or they may be catalyzed by microorganisms. For example, when microbes degrade organic compounds, they may transfer electrons from the compound to oxygen, converting the oxygen to water. Reductive dechlorination A process whereby bacteria remove a chlorine atom from a chlorinated compound and replace it with a hydrogen atom. Refractory Resistant to biodegradation. Remedial action objective A description of remedial goals for each medium of concern at a site, expressed in terms of the contaminant(s) of concern, exposure route(s) and receptor(s), and maximum acceptable exposure level(s). Remedial investigation (RI) A study carried out at a hazardous waste site covered under the Comprehensive Environmental Response, Compensation, and Liability Act to determine the extent of contamination and the risk it poses. Residence time The average amount of time a fluid spends during transport through a unit volume of subsurface or a laboratory vessel. Residual saturation The ratio of the volume of contaminant trapped in subsurface pore spaces to the total volume of pore spaces. If the level of contaminants trapped in the pore spaces is below residual saturation, the contaminants will not drain from the pores. For example, as oil moves through soil, it leaves oil trapped in the soil pores at residual saturation. Resource Conservation and Recovery Act (RCRA) A law that regulates monitoring, investigation, and corrective action at operating hazardous treatment, storage, and disposal facilities. RCRA will provide the framework for environmental investigations and cleanup at an estimated 5,000 operating facilities. Retardation The movement of a solute through a geologic medium at a velocity less than that of the ground water. Retardation is caused by sorption and other phenomena that separate a fraction of the solute mass from the bulk ground water. Retardation coefficient A measure of how quickly a contaminant moves through the ground compared to water. It is computed as the ratio of the total contaminant mass in a unit aquifer volume to the contaminant mass in solution.

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Alternatives for Ground Water Cleanup Risk assessment An evaluation of the potential for exposure to contaminants and the associated hazard. Risk-based concentration A contaminant concentration determined from an evaluation of the compound's overall risk to human health. S Safe Drinking Water Act The law, passed in 1974, that required the setting of standards to protect the public from exposure to contaminants in drinking water. Saturated zone The part of the subsurface that is beneath the water table and is saturated with water. Second Law of Thermodynamics A physical law that determines the direction of energy flow, such as the flow of heat from a warm body to a cooler body, or the irreversible conversion of work into heat caused by friction. Secondary substrate A substance that can be metabolized or transformed by microorganisms but that yields little or no energy for the organisms. Sedimentary rock A rock created from the consolidation of loose sediment that has accumulated in layers. Site characterization An attempt to identify the types and sources of contaminants present at a site and the site's hydrogeologic characteristics. Slurry wall A barrier constructed in the subsurface to prevent the spread of contaminants. Soil flushing The forced circulation (e.g., by use of injection and extraction wells) of water, steam, cosolvents, surfactants, or other fluids to enhance the recovery of contaminants (i.e., immiscible, dissolved, or adsorbed) from soil. Solidification/stabilization Processes that use cementing agents to mechanically bind subsurface contaminants and thereby reduce their rate of release. Sorption Refers to processes that remove solutes from the fluid phase and concentrate them on the solid phase of a medium. Source area The area at a contaminated site containing waste remaining in place. The source area may stretch beyond the original contaminant spill site; included in the committee's definition of source area are regions along the contaminant flow path where contaminants are present in precipitated or nonaqueous-phase liquid form. Source control A set of processes designed to prevent the spread of contaminants from the areas where the contaminants were originally spilled or areas where migrating contaminants have accumulated. Source of contamination A reservoir of contamination existing in a separate phase from water. The primary source of contamination is near

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Alternatives for Ground Water Cleanup the site where the contaminant was originally spilled. However, some types of contaminants may travel long distances as separate phases from the water or may precipitate in mineral form at considerable distances from the original spill site. Specific gravity The density of a liquid divided by the density of water. Steam stripping The use of steam to heat the subsurface and aid contaminant volatilization. Storage coefficient The volume of water released by pressure changes per unit area during pumping in a confined aquifer. This property influences the quantity of water that can be obtained by pumping. Stratigraphy The arrangement of rock strata in the subsurface. Substrate A substance metabolized or transformed by microorganisms. An "electron donor substrate" may be thought of as a food source; the microbes degrade it to provide energy for cell growth. An "electron acceptor substrate" is oxygen or another substance that microbes use in the process of digesting the electron donor substrate; the organisms transfer electrons from the electron donor to oxygen (or another electron acceptor), obtaining energy from the transfer. Sulfate reduction The conversion of sulfate to hydrogen sulfide by microorganisms. Because they can degrade hazardous compounds without using oxygen, sulfate-reducing bacteria can be important players in the subsurface, where the oxygen supply is often limited. Surface tension The tension at the surface between a liquid and its own vapor. Surfactant Soap or a similar amphiphilic substance that bonds to oil and other immiscible compounds to aid their transport in water. Surficial aquifer An aquifer near the earth's surface, in the most recent of geologic deposits. T Tailing The slow, nearly asymptotic decrease in contaminant concentration in water flushed through contaminated geologic material. Tracer test A method used to determine the flow of ground water or other fluids in the subsurface. It is conducted by injecting a nonreactive "tracer" chemical in the subsurface and monitoring its migration. Treatment train The combination of several remedial actions, such as a pump-and-treat system for ground water contamination combined with vacuum extraction for soil contamination. U Unsaturated zone The soil above the water table, where pores are partially (or largely) filled with air.

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Alternatives for Ground Water Cleanup V Vacuum extraction A method for removing volatile contaminants from the soil above the water table. Vadose zone The subsurface zone that extends between the ground surface and the water table and includes the capillary fringe overlying the water table. Vapor extraction See "Vacuum extraction." Viscosity An indicator of the ease with which a fluid will flow. The less viscous a fluid, the more easily it will flow. Volatile organic compound (VOC) An organic chemical that volatilizes (evaporates) relatively easily when exposed to air. Volatilization The transfer of a chemical from the liquid to the gas phase. W Water table The "top" of the subsurface zone that is saturated with ground water. More precisely, it is the surface in an aquifer at which pore water pressure is equal to atmospheric pressure. Water-table aquifer An aquifer in which the water table forms the upper boundary. Wellhead treatment Treatment of extracted water to remove chemicals prior to the water's distribution for drinking. This differs from complete cleanup of an aquifer, because only the water that will be used for drinking—not all the water at the contaminated site—is treated. Wettability The ability of a liquid traveling through the subsurface to form a film on the solid material in the subsurface. Z Zone of capture The area surrounding a pumping well that encompasses all areas or features that supply ground water recharge to the well. Zone of influence The area surrounding a pumping or recharging well within which the water table or potentiometric surface has been changed due to the well's pumping or recharge.