Environmental Health Center:
Glossary

Q

quality control (QC): The overall system of technical activities that measures the attributes and performance of a process, item, or service against defined standards to verify that they meet the stated requirements established by the customer; operational techniques and activities that are used to fulfill requirements for quality. The system of activities and checks used to ensure that measurement systems are maintained within prescribed limits, providing protection against “out of control” conditions and ensuring the results are of acceptable quality.

quench tank: A water-filled tank used to cool incinerator residue or hot materials from industrial processes.

quicklime: A material that is mostly calcium oxide (CaO) or calcium oxide in natural association with a lesser amount of magnesium oxide. Quicklime is capable of combining with water to form hydrated lime.

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R

radiation: Transmission of energy through space or any medium. Also known as radiant energy.

radical: A group of atoms that is capable of remaining unchanged during a series of chemical reactions. Such combinations (radicals) exist in the molecules of many organic compounds; sulfate (SO42­) is an inorganic radical.

radioactive decay: Spontaneous change in an atom by emission of charged particles and/or gamma rays; also known as radioactive disintegration and radioactivity.

radioactive waste: Any waste that emits energy as rays, waves, or streams of energetic particles. Radioactive materials are often mixed with hazardous waste, usually from nuclear reactors, research institutions, or hospitals.

radioisotopes: Chemical variants of an element with potentially oncogenic, teratogenic, and mutagenic effects on the human body.

radionuclide: Any man­made or natural element which emits radiation in the form of alpha or beta particles, or as gamma rays.

radius of vulnerability zone: The maximum distance from the point of release of a hazardous substance in which the airborne concentration could reach the level of concern under specified weather conditions.

radon: A colorless, naturally occurring gas formed by radioactive decay of radium atoms. Radon accumulating in basements and other areas of buildings without proper ventilation has been identified as a leading cause of lung cancer.

radon daughters/radon progeny: Short-lived radioactive decay products of radon that decay into longer-lived lead isotopes, The daughter isotopes can attach themselves to airborne dust and other particles and, if inhaled, damage to lining of the lung. Also known as radon decay products.

ranney collector: This water collector is constructed as a dug well from 12 to 16 feet (3.5 to 5 m) in diameter that has been sunk as a caisson near the bank of a river or lake. Screens are driven radially and approximately horizontally from this well into the sand and the gravel deposits underlying the river.

raw sewage: Untreated wastewater and its contents.

raw water: 1) Water in its natural state, prior to any treatment. 2) Usually the water entering the first treatment process of a water treatment plant.

reactivity: Refers to those hazardous wastes that are normally unstable and readily undergo violent chemical change but do not explode.

reaeration: Introduction of air into the lower layers of a reservoir. As the air bubbles form and rise through the water, the oxygen from the air dissolves into the water and replenishes the dissolved oxygen. The rising bubbles also cause the lower waters to rise to the surface where they take on oxygen from the atmosphere.

reagent: A pure chemical substance that is used to make new products or is used in chemical tests to measure, detect, or examine other substances.

recarbonation: A process in which carbon dioxide is bubbled into the water being treated to lower the pH. The pH may also be lowered by the addition of acid. Recarbonation is the final stage in the lime­soda ash softening process. This process converts carbonate ions to bicarbonate ions and stabilizes the solution against the precipitation of carbonate compounds.

receiving waters: All distinct bodies of water that receive runoff or wastewater discharges, such as streams, rivers, ponds, lakes, and estuaries.

receptor: 1) In biochemistry: a specialized molecule in a cell that binds a specific chemical with high specificity and high affinity. 2) In exposure assessment: an organism that receives, may receive, or has received environmental exposure to a chemical.

recharge: The process by which water is added to a zone of saturation, usually by percolation from the soil surface, e.g., the recharge of an aquifer.

reclamation: (In recycling) Restoration of materials found in the waste stream to a beneficial use which may be for purposes other than the original use.

recombinant bacteria: A microorganism whose genetic makeup has been altered by deliberate introduction of new genetic elements. The offspring of these altered bacteria also contain these new genetic elements, i.e. they “breed true.”

recombinant DNA: The new DNA that is formed by combining pieces of DNA from different organisms or cells.

recycling: Reusing materials and objects in original or changed forms rather than discarding them as wastes.

red tide: A proliferation of a marine plankton toxic and often fatal to fish, perhaps stimulated by the addition of nutrients. A tide can be red, green, or brown, depending on the coloration of the plankton.

reduction: Reduction is the addition of hydrogen, removal of oxygen, or the addition of electrons to an element or compound. Under anaerobic conditions (no dissolved oxygen present), sulfur compounds are reduced to odor-producing hydrogen sulfide (H2S and other compounds. The opposite of oxidation.

reformulated gasoline: Gasoline with a different composition from conventional gasoline (e.g., lower aromatics content) that cuts air pollutants.

regeneration: Manipulation of cells to cause them to develop into whole plants.

release: Any spilling, leaking, pumping, pouring, emitting, emptying, discharging, injecting, escaping, leaching, dumping, or disposing into the environment of a hazardous or toxic chemical or extremely hazardous substance.

rem: The unit of dose equivalent from ionizing radiation to the total body or any internal organ or organ system. A millirem (mrem)" is 1/1000 of a rem.

remediation: 1. Cleanup or other methods used to remove or contain a toxic spill or hazardous materials from a Superfund site; 2. for the Asbestos Hazard Emergency Response program, abatement methods including evaluation, repair, enclosure, encapsulation, or removal of greater than 3 linear feet or square feet of asbestos-containing materials from a building.

remote sensing: The capture of information about the Earth from a distant vantage point. The term is often associated with satellite imagery but also applies to aerial photography, airborne digital sensors, ground-based detectors, and other devices.

reservoir: Any natural or artificial holding area used to store; regulate, or control water.

residual: Amount of a pollutant remaining in the environment after a natural or technological process has taken place, e.g., the sludge remaining after initial wastewater treatment, or particulates remaining in air after it passes through a scrubbing or other process.

residual risk: The extent of health risk from air pollutants remaining after application of the Maximum Achievable Control Technology (MACT).

residue: The dry solids remaining after the evaporation of a sample of water or sludge. Also see total dissolved solids

resistance: For plants and animals, the ability to withstand poor environmental conditions or attacks by chemicals or disease. May be inborn or acquired.

resource recovery: The extraction of useful materials or energy from solid waste. Such materials can include paper, glass, and metals that can be reprocessed for re­use. Resource recovery also is employed in pollution prevention.

respiration: The process in which an organism uses oxygen for its life processes and gives off carbon dioxide.

restoration: Measures taken to return a site to pre­violation conditions.

retardation: Preferential retention of contaminant movement in the subsurface resulting from adsorptive processes or solubility differences. saturated zone: the zone in which all the voids in the rock or soil are filled with water at greater than atmospheric pressure. The water table is the top of the saturated zone in an unconfined aquifer.

retrofit: Addition of a pollution control device on an existing facility without making major changes to the generating plant.

reuse: Using a product or component of municipal solid waste in its original form more than once, e.g., refilling a glass bottle that has been returned or using a coffee can to hold nuts and bolts.

reversible effect: An effect which is not permanent, especially adverse effects which diminish when exposure to a toxic chemical is ceased.

ribonucleic acid (RNA): A molecule that carries the genetic message from DNA to a cellular protein-producing mechanisms.

riffle: A rocky shoal or sandbar lying just below the surface of a waterway. The choppy water created by such sand bars and shoals is also referred to as a riffle.

rill: A small channel eroded into the soil surface by runoff, rills easily can be smoothed out (obliterated) by normal tillage.

riparian rights: A doctrine of state water law under which a land owner is entitled to use the water on or bordering his property, including the right to prevent diversion or misuse of upstream waters. Riparian land is land that borders on surface water.

risk: A measure of the probability that damage to life, health, property, and/or the environment will occur as a result of a given hazard.

risk assessment: A qualitative or quantitative evaluation of the environmental and/or health risk resulting from exposure to a chemical or physical agent (pollutant); combines exposure assessment results with toxicity assessment results to estimate risk.

risk factor: A characteristic (e.g., race, sex, age, obesity) or variable (e.g., smoking, exposure) associated with increased chance of toxic effects. Some standard risk factors used in general risk assessment calculations include average breathing rates, average weight, and average human life span.

risk management: Decisions about whether an assessed risk is sufficiently high to present a public health concern and about the appropriate means for control of a risk judged to be significant. The process of evaluating and selecting alternative regulatory and non-regulatory responses to risk. The selection process necessarily requires the consideration of legal, economic, and behavioral factors.

river basin: The land area drained by a river and its tributaries.

rodenticide: A chemical or agent used to destroy rats or other rodent pests, or to prevent them from damaging food, crops, etc.

rotary kiln incinerator: An incinerator with a rotating combustion chamber that keeps waste moving, thereby allowing it to vaporize for easier burning.

rough fish: Fish not prized for eating, such as gar and suckers. Most are more tolerant of changing environmental conditions than game species.

route of exposure: The avenue by which a chemical comes into contact with an organism (e.g., inhalation, ingestion, dermal contact, injection).

run-off: That part of precipitation, snow melt, or irrigation water that runs off the land into streams or other surface water. It can carry pollutants from the air and land into the receiving waters.

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S

safe: Condition of exposure under which there is a “practical certainty” that no harm will result in exposed individuals.

safe water: Water that does not contain harmful bacteria, or toxic materials or chemicals. Water may have taste and odor problems, color and certain mineral problems and still be considered safe for drinking.

safe yield: The annual amount of water that can be taken from a source or supply over a period of years without depleting that source beyond its ability to be replenished naturally in "wet years".

salinity: 1)The relative concentration of dissolved salts, usually sodium chloride, in a given water.. 2) A measure of the concentration of dissolved mineral substances in water.

salt water intrusion: The invasion of fresh surface or ground water by salt water. If it comes from the ocean it may be called sea water intrusion.

salts: Minerals that water picks up as it passes through the air, over and under the ground, or from households and industry.

salvage: The utilization of waste materials.

sanctions: Actions taken by the federal government for failure to plan or implement a State Improvement Plan (SIP). Such action may be include withholding of highway funds and a ban on construction of new sources of potential pollution.

sand filters: Devices that remove some suspended solids from sewage. Air and bacteria decompose additional wastes filtering through the sand so that cleaner water drains from the bed.

sanitary sewer: A sewer that transports only wastewaters (from domestic residences and/or industries) to a wastewater treatment plant.

sanitary water: Water discharged from restrooms, showers, food preparation facilities, or other nonindustrial operations; also known as “gray water.”

saprophytes: Organisms living on dead or decaying organic matter that help natural decomposition of organic matter in water.

saturated zone: The area below the water table where all open spaces are filled with water.

scrap: Materials discarded from manufacturing operations that may be suitable for reprocessing.

scrubber: An air pollution device that uses a spray of water or reactant or a dry process to trap pollutants in emissions.

secondary treatment: The second step in most publicly owned waste treatment systems in which bacteria consume the organic parts of the waste. It is accomplished by bringing together waste, bacteria, and oxygen in trickling filters or in the activated sludge process. This treatment removes floating and settleable solids and about 90 percent of the oxygen-demanding substances and suspended solids. Disinfection is the final stage of secondary treatment.

sedges: Plants of the family Cyperacae that resemble grasses, but have solid stems

sediment: Topsoil, sand, and minerals washed from the land into water, usually after rain or snow melt. Sediments collecting in rivers, reservoirs, and harbors can destroy fish and wildlife habitat and cloud the water so that sunlight cannot reach aquatic plants. Loss of topsoil from farming, mining, or building activities can be prevented through a variety of erosion-control techniques.

seepage: Percolation of water through the soil from unlined canals, ditches, laterals, watercourses, or water storage facilities.

sentinel well: A groundwater monitoring well situated between a sensitive receptor downgradient and the source of a contaminant plume upgradient. Contamination should be first detected in the sentinel well which serves as a warning that contamination may be moving closer to the receptor. The sentinel well should be located far enough upgradient of the receptor to allow enough time before the contamination arrives at the receptor to initiate other measures to prevent contamination from reaching the receptor, or in the case of a supply well, provide for an alternative water source.

septic system: An onsite system designed to treat and dispose of domestic sewage. A typical septic system consists of a tank that receives waste from a residence or business and a system of tile lines or a pit for disposal of the liquid effluent (sludge) that remains after decomposition of the solids by bacteria in the tank and must be pumped out periodically.

sequestration: The inhibition or stoppage of normal ion behavior by combination with added materials, especially the prevention of metallic ion precipitation from solution by formation of a coordination complex with a phosphate.

settling tank: A holding area for wastewater, where heavier particles sink to the bottom for removal and disposal.

sewage: The used water and solids that flow from homes through sewers to a wastewater treatment plant. The preferred term is wastewater.

sewer: An underground system of conduits (pipes and/or tunnels) that collect and transport wastewaters and/or runoff; gravity sewers carry free-flowing water and wastes; pressurized sewers carry pumped wastewaters under pressure.

shock load: The arrival at a water treatment plant of raw water containing unusual amounts of algae, colloidal mater, color, suspended solids, turbidity, ore other pollutants.

short circuiting: The entry of ambient air into an extraction well (used for SVE and bioventing) without first passing through the contaminated zone. Short circuiting may occur through utility trenches, incoherent well or surface seals, or layers of high permeability geologic materials. Also when some of the water in tanks or basins flows faster than the rest; usually undesirable since it may result in shorter contact, reaction, or settling times in comparison with the calculated or presumed detention times.

silt: Sedimentary materials composed of fine or intermediate sized mineral particles.

silviculture: Management of forest land for timber.

sink: A place in the environment where a compound or material collects. See reservoir.

sinking: Controlling oil spills by using an agent to trap the oil and sink it to the bottom of the body of water where the agent and the oil are biodegraded.

skimming: Using a machine to remove oil or scum from the surface of the water.

slake: To mix with water with a true chemical combination (hydrolysis) taking place, such as in the slaking of lime.

slow sand filtration: A process involving passage of raw water through a bed of sand at low velocity (generally less than 0.4 m/h) resulting in substantial particulate removal by physical and biological mechanisms.

sludge: A semi-solid residue from any of a number of air or water treatment processes; can be a hazardous waste.

slurry: A watery mixture or suspension of insoluble (not dissolved) matter; a thin watery mud or any substance resembling it (such as a grit slurry or a lime slurry).

smelter: A facility that melts or fuses ore, often with an accompanying chemical change, to separate its metal content. Emissions cause pollution. “Smelting” is the process involved.

smog: Dust, smoke, or chemical fumes that pollute the air and make hazy, unhealthy conditions (literally, the word is a blend of smoke and fog). Automobile, truck, bus, and other vehicle exhausts and particulates are usually trapped close to the ground, obscuring visibility and contributing to a number of respiratory problems.

soft water: Water having a low concentration of calcium and magnesium ions. According to U.S. Geological Survey guidelines, soft water is water having a hardness of 60 milligrams per liter or less.

soil adsorption field: A sub-surface area containing a trench or bed with clean stones and a system of piping through which treated sewage may seep into the surrounding soil for further treatment and disposal.

soil erodibility: A measure of the soil's susceptibility to raindrop impact, runoff and other erosional processes.

soil profile: A vertical section of the earth's highly weathered upper surface often showing several distinct layers, or horizons.

sole source aquifer: An aquifer that supplies 50 percent or more of the drinking water of an area.

solid waste: As defined under RCRA, any solid, semi-solid, liquid, or contained gaseous materials discarded from industrial, commercial, mining, or agricultural operations, and from community activities. Solid waste includes garbage, construction debris, commercial refuse, sludge from water supply or waste treatment plants, or air pollution control facilities, and other discarded materials. Solid Waste Management Facility: Any disposal or resource recovery system; any system, program, or facility for resource conservation; any facility for the treatment of solid wastes.

solidification and stabilization: Removal of wastewater from a waste or changing it chemically to make it less permeable and susceptible to transport by water.

solution: A liquid mixture of dissolved substances. In a solution it is impossible to see all the separate parts.

soot: Carbon dust formed by incomplete combustion.

source reduction: The design, manufacture, purchase, or use of materials (such as products and packaging) to reduce the amount or toxicity of garbage generated. Source reduction can help reduce waste disposal and handling charges because the costs of recycling, municipal composting, landfilling, and combustion are avoided. Source reduction conserves resources and reduces pollution.

sparge: Injection of air below the water table to strip dissolved volatile organic compounds and/or oxygenate the groundwater to facilitate aerobic biodegradation of organic compounds.

species: A reproductively isolated aggregate of interbreeding organisms.

sphagnum: Any of various pale or ashy mosses of the genus Sphagnum, the decomposed remains of which form peat.

spore: The reproductive body of an organism which is capable of giving rise to a new organism either directly or indirectly. A viable (able to live and grow) body regarded as the resting stage of an organism. A spore is usually more resistant to disinfectants and heat than most organisms.

sprawl: Unplanned development of open land.

spray tower scrubber: A device that sprays alkaline water into a chamber where acid gases present to aid in the neutralizing of the gas.

stabilization: Conversion of the active organic matter in sludge into inert, harmless material.

stable air: A motionless mass of air that holds instead of dispersing pollutants.

stack effect: Air, as in a chimney, that moves upward because it is warmer than the ambient atmosphere.

stagnation: Lack of motion in a mass of air or water that holds pollutants in place.

state emergency response commission (SERC): The agency appointed by the Governor to oversee the administration of EPCRA at the state level. This commission designates and appoints members to LEPCs and reviews emergency response plans for cities and counties.

state implementation plans (SIP): EPAapproved state plans for the establishment, regulation, and enforcement of air pollution standards.

stationary source: A fixed­site producer of pollution, mainly power plants and other facilities using industrial combustion processes.

sterilization: The removal or destruction of all microorganisms, including pathogenic and other bacteria, vegetative forms and spores. Compare with disinfection.

storm sewer: A sewer that collects and transports surface runoff to a discharge point (infiltration basin, receiving stream, treatment plant).

stratification: The formation of separate layers (of temperature, plant, or animal life) in a lake or reservoir. Each layer has similar characteristics such as all water in the layer has the same temperature.

stratosphere: The portion of the atmosphere 10-to-25 miles above the earth´s surface.

stratum: A horizontal layer of geologic material of similar composition, especially one of several parallel layers arranged one on top of another.

streambed: The channel through which a natural stream or river runs or once ran through.

strip cropping: A crop production system that involves planting alternating strips of row crops and close-growing forage crops; the forage strips intercept and slow runoff from the less protected row crop strips.

strip mining: A process that uses machines to scrape soil or rock away from mineral deposits just under Earth´s surface.

submerged aquatic vegetation (SAV): Aquatic vegetation, such as sea grasses, that cannot withstand excessive drying and therefore live with their leaves at or below the water surface. SAVs provide an important habitat for young fish and other aquatic organisms.

sulfur dioxide: A pungent, colorless, gaseous pollutant formed primarily by the combustion of fossil fuels.

supercritical water: A type of thermal treatment using moderate temperatures and high pressures to enhance the ability of water to break down large organic molecules into smaller, less toxic ones. Oxygen injected during this process combines with simple organic compounds to form carbon dioxide and water.

superfund: The program operated under the legislative authority of CERCLA and SARA that funds and carries out EPA solid waste emergency and long-term removal and remedial activities. These activities include establishing the National Priorities List, investigating sites for inclusion on the list, determining their priority, and conducting and/or supervising the cleanup and other remedial actions.

supernatant: Liquid removed from settled sludge. Supernatant commonly refers to the liquid between the sludge on the bottom and the water surface of a basin or container.

supersaturated: An unstable condition of a solution (water) in which the solution contains a substance at a concentration greater than the saturation concentration for the substance.

surface impoundment: Treatment, storage, or disposal of liquid hazardous wastes in ponds.

surface runoff: Precipitation, snow melt, or irrigation in excess of what can infiltrate the soil surface and be stored in small surface depressions; runoff is a major transporter of non­point source pollutants.

surface water: All water naturally open to the atmosphere (rivers, lakes, reservoirs, ponds, streams, impoundments, seas, estuaries, etc.) and all springs, wells, or other collectors directly influenced by surface water.

suspended solids: I)Solids that either float on the surface or are suspended in water or other liquids, and which are largely removable by laboratory filtering. 2) The quantity of material removed from water in a laboratory test.

sustainable agriculture: Environmentally friendly methods of farming that allow the production of crops or livestock without damage to the farm as an ecosystem, including effects on soil, water supplies, biodiversity, or other surrounding natural resources. The concept of sustainable agriculture is an “intergenerational” one in which we pass on a conserved or improved natural resource base instead of one which has been depleted or polluted. Terms often associated with farms or ranches that are self-sustaining include “low-input,” organic, “ecological,” “biodynamic,” and “permaculture.”

swamp: A type of wetland dominated by woody vegetation but without appreciable peat deposits. Swamps may be fresh or salt water and tidal or non-tidal. (See ‘wetlands’)

symbiosis: The relationship of two or more different organisms in a close association that may be but is not necessarily of benefit to each.

synergism: The cooperative action of two or more organisms producing a greater total result than the sum of their independent effects; chemicals or muscles in synergy enhance the effectiveness of one another beyond what an individual could have produced.

systemic effects: Effects observed at sites distant from the entry point of a chemical due to its absorption and distribution into the body.

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T

tail water: The runoff of irrigation water from the lower end of an irrigated field.

tailings: Residue of raw material or waste separated out during the processing of crops or mineral ores.

tailpipe standards: Emissions limitations applicable to engine exhausts from mobile sources.

teratogenesis: The induction of nonhereditary congenital malformations (birth defects) in a developing fetus by exogenous factors acting in the womb; interference with normal embryonic development.

terrace: A broad channel, bench, or embankment constructed across the slope to intercept runoff and detain or channel it to protected outlets, thereby reducing erosion from agricultural areas.

tertiary treatment: An enhancement of normal sewage treatment operations to provide water of potable quality using further chemical and physical treatment; the highest drinking water standard achieved in the U.S.

thermal stratification: The formation of layers of different temperatures in a lake or reservoir.

thermocline: The middle layer in a thermally stratified lake or reservoir. In this layer there is a rapid decrease in temperature with depth. Also called the METALIMNION.

threshold: The lowest dose of a chemical at which a specified measurable effect is observed and below which it is not observed.

threshold level: Time-weighted average pollutant concentration values, exposure beyond which is likely to adversely affect human health. (See ‘environmental exposure’)

tidal marsh: Low, flat marshlands traversed by channels and tidal hollows, subject to tidal inundation; normally, the only vegetation present is salttolerant bushes and grasses.

tillage: Plowing, seedbed preparation, and cultivation practices.

time-weighted average (TWA): In air sampling, the average air concentration of contaminants during a given period.

tolerance: Permissible residue level for pesticides in raw agricultural produce and processed foods. Whenever a pesticide is registered for use on a food or feed crop, a tolerance must be established. EPA establishes the tolerance levels, which are enforced by the Food and Drug Administration and the Department of Agriculture. Tonnage: The amount of waste that a landfill accepts, usually expressed as tons per month. The rate at which a landfill accepts waste is limited by the landfill´s permit.

tonnage: The amount of waste that a landfill accepts, usually expressed in tons per month. The rate at which a landfill accepts waste is limited by the landfill´s permit.

toxaphene: A chemical that causes adverse health effects in domestic water supplies and also is toxic to freshwater and marine aquatic life.

toxic chemical: Substances that can cause severe illness, poisoning, birth defects, disease, or death when ingested, inhaled, or absorbed by living organisms.

toxic cloud: Airborne plume of gases, vapors, fumes, or aerosols containing toxic materials.

toxic pollutants: Materials contaminating the environment that cause death, disease. birth defects in organisms that ingest or absorb them. The quantities and length of exposure necessary to cause these effects can vary widely.

toxic release inventory (TRI): A database of annual toxic releases from certain manufacturers compiled from EPCRA Section 313 reports. Manufacturers must report annually to EPA and the states the amounts of almost 350 toxic chemicals and 22 chemical categories that they release directly to air, water, or land, inject underground, or transfer to off-site facilities. EPA compiles these reports and makes the information available to the public under the “Community Right­to­Know” portion of the law.

toxic substance: A chemical or mixture that can cause illness, death, disease, or birth defects. The quantities and exposures necessary to cause these effects can vary widely. Many toxic substances are pollutants and contaminants in the environment.

toxic waste: A waste that can produce injury if inhaled, swallowed, or absorbed through the skin.

transboundary pollutants: Air pollution that travels from one jurisdiction to another, often crossing state or international boundaries.

transient water system: A non-community water system that does not serve 25 of the same nonresident persons per day for more than six months per year. Also called a transient non-community water system (TNCWS).

transpiration: The process by which water vapor is lost to the atmosphere from living plants. The term can also be applied to the quantity of water thus dissipated.

treated wastewater: Wastewater that has been subjected to one or more physical, chemical, and biological processes to reduce its pollution of health hazard.

treatment plant: A structure built to treat wastewater before discharging it into the environment.

tributary: A stream or other body of water, surface or underground, which intermittently contributes its water in small quantities to another larger stream or body of water.

trichloroethane (TCE): A stable, low boiling-point colorless liquid, toxic if inhaled. Used as a solvent or metal decreasing agent, and in other industrial applications.

trickle irrigation: Method in which water drips to the soil from perforated tubes or emitters.

trickling filter: A coarse treatment system in which wastewater is trickled over a bed of stones or other material covered with bacteria that break down the organic waste and produce clean water.

trihalomethane (THM): One of a family of organic compounds named as derivative of methane. THMs are generally by-products of chlorination of drinking water that contains organic material.

troposphere: The layer of the atmosphere closest to the earth's surface.

tundra: A type of ecosystem dominated by lichens, mosses, grasses, and woody plants. Tundra is found at high latitudes (arctic tundra) and high altitudes (alpine tundra). Arctic tundra is underlain by permafrost and is usually saturated. (See ‘wetlands’)

turbidity: The cloudy appearance of water caused by the presence of suspended and colloidal matter. In the waterworks field, a turbidity measurement is used to indicate the clarity of water. Technically, turbidity is an optical property of the water based on the amount of light reflected by suspended particles. Turbidity cannot be directly equated to suspended solids because white particles reflect more light than dark-colored particles and many small particles will reflect more light than an equivalent large particle.

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U

ultraviolet rays: Radiation from the sun that can be useful or potentially harmful. UV rays from one part of the spectrum (UV-A) enhance plant life and are useful in some medical and dental procedures; UV rays from other parts of the spectrum (UV-B) can cause skin cancer or other tissue damage. The ozone layer in the atmosphere partly shields us from ultraviolet rays reaching the earth's surface.

underground storage tank (UST): A tank and any underground piping connected to the tank that has 10% or more of its volume (including pipe volume) beneath the surface of the ground. USTs are designed to hold gasoline, other petroleum products, and hazardous materials.

unsaturated: The characteristic of a carbon atom in a hydrocarbon molecule that shares a double bond with another carbon atom.

uranium mill-tailings waste piles: Licensed active mills with tailings piles and evaporation ponds created by acid or alkaline leaching processes.

urban runoff: Storm water from city streets and adjacent domestic or commercial properties that carries pollutants of various kinds into the sewer systems and receiving waters.

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V

vadose zone: The zone between land surface and the water table within which the moisture content is less than saturation (except in the capillary fringe) and pressure is less than atmospheric. Soil pore spaces also typically contain air or other gases. The capillary fringe is included in the vadose zone.

vapor: The gas given off by substances that are solids or liquids at ordinary atmospheric pressure and temperatures. Vapor Dispersion: The movement of vapor clouds or plumes in the air due to wind, gravity, spreading, and mixing.

vapor density: The amount of mass of a vapor per unit volume of the vapor.

vapor dispersion: The movement of vapor clouds in air due to wind, thermal action, gravity spreading, and mixing.

vapor plumes: Flue gases visible because they contain water droplets.

vector: 1. An organism, often an insect or rodent, that carries disease. 2. Plasmids, viruses, or bacteria used to transport genes into a host cell. A gene is placed in the vector; the vector then "infects" the bacterium.

vegetative controls: Non-point source pollution control practices that involve plants (vegetative cover) to reduce erosion and minimize the loss of pollutants.

vinyl chloride: A chemical compound, used in producing some plastics, that is believed to be oncogenic.

virgin materials: Resources extracted from nature in their raw form, such as timber or metal ore.

virus: The smallest form of microorganisms capable of causing disease. Especially, a virus of fecal origin that is infectious to humans by waterborne transmission.

viscosity: A measure of the internal friction of a fluid that provides resistance to shear within the fluid. The greater the forces of internal friction (i.e. the greater the viscosity), the less easily the fluid will flow.

volatile acids: Acids produced during digestion. Fatty acids which are soluble in water and can be steam-distilled at atmospheric pressure. Also called “organic acids.” Volatile acids are commonly reported as equivalent to acetic acid.

volatile liquids: Liquids which easily vaporize or evaporate at room temperature.

volatile organic compound (VOC): Any organic compound which evaporates readily to the atmosphere. VOCs contribute significantly to photochemical smog production and certain health problems.

volatile solids: Those solids in water or other liquids that are lost on ignition of the dry solids at 550 degrees C.

vortex: A revolving mass of water which forms a whirlpool. This whirlpool is caused by water flowing out of a small opening in the bottom of a basin or reservoir. A funnelshaped opening is created downward from the water surface.

vulnerable zone: An area over which the airborne concentration of a chemical accidentally released could reach the level of concern.

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W

waste exchange: Arrangement in which companies exchange their wastes for the benefit of both parties.

waste load allocation: The maximum load of pollutants each discharger of waste is allowed to release into a particular waterway. Discharge limits are usually required for each specific water quality criterion being, or expected to be, violated. The portion of a stream's total assimilative capacity assigned to an individual discharge.

waste reduction: Using source reduction, recycling, or composting to prevent or reduce waste generation.

waste stream: The total flow of solid waste from homes, businesses, institutions, and manufacturing plants that are recycled, burned, or disposed of in landfills, or segments thereof such as the "residential waste stream" or the “recyclable waste stream.”

waste water: Water carrying wastes from homes, businesses and industries that is a mixture of water and dissolved or suspended solids.

water budget: A summation of inputs, outputs, and net changes to a particular water resource system over a fixed period. (Also, water balance model).

water cycle: The process, also known as the hydrologic cycle, in which water travels in a sequence from the air through condensation to the earth as precipitation and back to the atmosphere by evaporation.

water quality criteria: Levels of water quality expected to render a body of water suitable for its designated use. Criteria are based on specific levels of pollutants that would make the water harmful if used for drinking, swimming, farming, fish production, or industrial processes.

water quality standard (WQS): The combination of a designated use and the maximum concentration of a pollutant which will protect that use for any given body of water. For example, in a trout stream, the concentration of iron should not exceed 1 mg/l.

water solubility: The maximum concentration of a chemical compound which can result when it is dissolved in water. If a substance is water soluble, it can very readily disperse through the environment.

water storage pond: An impound for liquid wastes, so designated as to accomplish some degree of biochemical treatment of the wastes.

water table: The level of ground water. The upper surface of the zone of saturation of groundwater above an impermeable layer of soil or rock (through which water cannot move) as in an unconfined aquifer. This level can be very near the surface of the ground or far below it.

water vapor: Water diffused as a gas in the atmosphere.

watershed: The land area that drains into a stream. An area of land that contributes runoff to one specific delivery point; large watersheds may be composed of several smaller "subsheds", each of which contributes runoff to different locations that ultimately combine at a common delivery point.

weathering: The process during which a complex compound is reduced to its simpler component parts, transported via physical processes, or biodegraded over time.

weir (weer): 1) A wall or plate placed in an open channel and used to measure the flow of water. The depth of the flow over the weir can be used to calculate the flow rate, or a chart or conversion table may be used. 2) A wall or obstruction used to control flow (from settling tanks and clarifiers) to assure uniform flow rate and avoid short-circuiting.

well: A bored, drilled, or driven shaft, or a dug hole whose depth is greater than the largest surface dimension and whose purpose is to reach underground water supplies or oil, or to store or bury fluids below ground.

wellhead: The area immediately surrounding the top of a well, or the top of the well casing.

wetlands: Any number of tidal and nontidal areas characterized by saturated or nearly saturated soils most of the year that form an interface between terrestrial (land-based) and aquatic environments; include freshwater marshes around ponds and channels (rivers and streams), brackish and salt marshes; other common names include swamps and bogs.

wildlife refuge: An area designated for the protection of wild animals, within which hunting and fishing are either prohibited or strictly controlled.

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X

xenobiote: Any biotum displaced from its normal habitat; a chemical foreign to a biological system.

Y

yield: The quantity of water (expressed as a rate of flowGPM, GPH, GPD, or total quantity per year) that can be collected for a given use from surface or groundwater sources. The yield may vary with the use proposed, with the plan of development, and also with economic considerations.

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Z

zone of saturation: The soil or rock located below the top of the groundwater table. By definition, the zone of saturation is saturated with water. Also see water table.

zooplankton: Small, usually microscopic animals(such as protozoans), found in lakes and reservoirs.

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