table A set of data arranged in rows and columns.

taiga Flat, marshy, subartic forests, usually of spruce, firs, or pine trees; the area between the tundra and the steppe in Russia and between the tundra and deciduous forest or grassland of North America.

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.

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

take-or-pay A contract provision obligating a purchaser to pay for a commodity whether or not delivery is taken.

tampering Adjusting, negating, or removing pollution control equipment on a motor vehicle.

tangible loss Items of loss or damage that can be reasonably given a monetary value based on or derived from the normal criteria of the marketplace.

tapered aeration An aeration method whereby the quantity of air added varies along the aeration basin with a maximum at the head end and a minimum at the outlet end.

target (organism) The specific organism or group of organisms under test.

tariff The schedule of a utility’s rates and charges.

taxon Any taxonomic unit, from biotype or ecotype to phylum or kingdom.

taxonomy The study of the principles and practice for the orderly classification of organisms.

TCDD A by-product of pesticide manufacture; a toxic compound that is carcinogenic and teratogenic in certain animals.

TCE Trichloroethane.

TDS Total dissolved solids.

technical assistance grant (TAG) As part of the Superfund program, technical assistance grants of up to $50,000 are provided to citizens' groups to obtain assistance in interpreting information related to cleanups at Superfund sites or those proposed for the National Priorities List.

technology-based limitations Industry-specific effluent limitations applied to a discharge when it will not cause a violation of water quality standards at low stream flows. Usually applied to discharges into large rivers.

technology-based standards Effluent limitations applicable to direct and indirect sources which are developed on a category-by-category basis using statutory factors, not including water-quality effects.

telecommunication The process of transmitting or receiving information over a distance by any electrical or electromagnetic medium. Information may take the form of voice, video, or data.

teleconference Simultaneous conference to multiple sites distributed via audio (telephone or other audio). Satellite videoconferences and videoconferences using compressed video are sometimes referred to as "teleconferences." To distinguish more accurately between these frequently used terms, using the term which uniquely describes the communication is preferred.

telemedicine Use of telecommunications technology for medical diagnosis and patient care when the provider and client are separated by distance. Telemedicine includes pathology, radiology, and patient consultation from the distance.

telemetry The electrical link between the transmitter and the receiver. Telephone lines are commonly used to serve as the electrical line.

teleost Pertaining to ordinary (bony) fishes, exclusive of sharks, lampreys, gar, sturgeons, and a few others.

temperature A measure of the internal energy of molecular motion in a substance. Two scales are commonly used. The Fahrenheit and Centigrade temperature scales establish the freezing of water at 32/0 degrees respectively and boiling point at 212/100 degrees respectively. The Fahrenheit scale is used most frequently in the US and Centigrade throughout the rest of the world.

temperature sensor A device that opens and closes a switch in response to changes in temperature. This device might be a metal contact, a thermocouple that generates minute electrical current proportional to the difference in heat, or a variable resistor whose value changes in response to changes in temperature. Also called a heat sensor.

temporal variations Some characteristics of groundwater vary depending upon the time of year. For example, water levels in shallow aquifers in the eastern United States for the summer and winter months will be lower than levels in the spring and fall when most of the yearly precipitation occurs. Groundwater managers need to be aware of these variations when assessing groundwater.

temporary hardness Presence of calcium and magnesium bicarbonates that precipitate from water at 100° C.

temporary restraining order (TRO) A court-enforced cease and desist orders that is brought to control an emergency situation.

temporary threshold shift (TTS) A temporary increase in hearing threshold level, after exposure to noise, that reverts to the pre-exposure hearing threshold level.

tensiometer A device used to measure the negative pressure (or tension) with which water is held in the soil; a porous, permeable ceramic cup connected through a tube to a manometer or vacuum gage.

Ten States Standards The design standards established by the Great Lakes-Upper Mississippi River Board of State Public Health and Environmental Managers for sewage collection systems and treatment works that were the first to be widely accepted nationally. The Board consisted of ten states (Illinois, Iowa, Minnesota, New York Pennsylvania, Indiana, Michigan, Missouri, Ohio, and Wisconsin).

teratogen Any substance that tends to cause birth defects after conception.

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

teratogenicity The capacity of a physical or chemical agent to cause teratogenesis in offspring.

terminal reservoir An impoundment providing end storage of water prior to treatment.

terminated Ended; adjudicated.

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. Also terracing.

terrain A tract of land; its physical features with special emphasis on bedrock geology.

terrestrial Of the land, continents, and/or dry ground, as opposed to aquatic.

territoriality Any active behavioral mechanism that spaces organisms or groups apart from one another; usually shown by vertebrates.

territory That area in which an animal actively defends.

tertiary treatment Advanced waste treatment, usually including chemical clarification and filtration.

tetratogenesis The introduction of nonhereditary birth defects in a developing fetus by exogenous factors such as physical or chemical agents acting in the womb to interfere with normal embryonic development.

teratogenic Capable of interfering with the development of a fetus, causing birth defects.

thallophyte A plant with no true stem, roots, or leaves; includes the algae and fungi.

t-helper cells T-lymphocytes with the specific capacity to help other cells, such as b-lymphocytes, to make antibodies. T-helper cells are also required for the induction of other t-lymphocyte activities.

theme [ GIS] A grouping of coverages organized by subject type such as vegetation. In ArcView theme is the same as coverage.

therapeutic index The ratio of the dose required to produce toxic or lethal effect to dose required to produce nonadverse or therapeutic response.

thermal radiation electromagnetic radiation emitted from an explosion in the form of heat and light.

thermal system insulation (TSI) Asbestos-containing material applied to pipes, fittings, boilers, breeching, tanks, ducts, or other interior structural components to prevent heat loss or gain or water condensation.

thermal pollution The excessive raising or lowering of water temperatures above or below normal seasonal ranges in streams, lakes, estuaries, or oceans as a result of the discharge of hot or cold effluents into such waters.

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

thermal treatment Use of elevated temperatures to treat organic matter and hazardous wastes. (See incineration; pyrolysis.)

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 metaliminion.

thermocouple A heat-sensing device made of two conductors of different metals joined at their ends. An electric current is produced when there is a difference in temperature between the ends.

thermodynamics In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.)

thermoelectric power water use Water used in the process of the generation of thermoelectric power. The water may be obtained from a public water system supply or may be self supplied. See also public water system and self-supplied water.

thermogalvanic corrosion Galvanic corrosion resulting from temperature differences at two points.

thermolabile Easily altered or destroyed by heat, usually of about 50°C or higher.

thermophile An organism growing best at temperatures of about 50°C or higher.

thermophilic bacteria A group of bacteria that thrive in temperatures above 113°F.

thermotaxis Directional movement induced by heat; moving toward or away from a heat source.

thickening A process that separates supernatent from sludge.

thief hole A digester sampling well.

thigmotropic A response of an organism to touch or mechanical stimulation.

THM Trihalomethane.

THM precursor See precursor, trihalomethane.

threshold 1) The lowest limit at which a certain phenomenon will occur. 2) 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.)

threshold limit value (TLV) The average concentration of toxic gas or any other substance to which a normal person can be exposed without injury during an average work week.

threshold limit value-ceiling (TLV-C) The concentration that should not be exceeded during any part of the working exposure. In conventional industrial hygiene practice, if instantaneous monitoring is not feasible, the TLV-C can be expressed by sampling over a 15-minute period, except for those substances that may cause immediate irritation when exposures are short.

threshold limit value-short term exposure limit (TLV-STEL) 1) Defined as a 15-minute TWA exposure which should not be exceeded at any time during a workday even if the eight-hour TWA is within the TLV-TWA. Exposures above the TLV-TWA up to the STEL should not be longer than 15 minutes and should not occur more than four times per day. There should be at least 60 minutes between successive exposures in this range. An averaging period other than 15 minutes may be recommended when it is warranted by observed biological effects. 2) The concentration to which nearly all workers can be exposed continuously for a short period of time without suffering from irritation, chronic or irreversible tissue damage, or narcosis of a sufficient degree to increase the likelihood of accidental injury, impair self rescue or materially reduce work efficiency. It supplements the time­weighted average limit where there are recognized acute effects from a substance whose toxic effects have been reported from high short-term exposures in either humans or animals.

threshold odor An odor of a sample that can just be detected after successive dilutions with an odorless medium. Also called odor threshold.

threshold odor number The greatest dilution of a sample with odor-free water that still yields a just-detectable odor.

threshold temperature A temperature that denotes the boundary condition fora specific event. For example, a crop specific temperature below which the growth of that crop is minimal.

threshold treatment

thrust block A mass of concreter or similar material appropriately place around a pipe to prevent movement when the pipe is carrying water. Usually placed at bends and valve structures. See restraining gland.

thyroid blocking agent A pill or liquid containing non-radioactive iodine which, when taken before or immediately after exposure to radioactive iodine, saturates the thyroid gland to prevent excessive uptake of radioactive iodine.

tidal marsh Marsh land periodically inundated by tidal oceanic or estuarine water.

tier 1 violation A more serious violation, including failure to comply with an MCL, failure to comply with prescribed treatment techniques, and failure to meet variance or exemption schedules.

tier 2 violation A less serious violation, including failure to comply with monitoring requirements, failure to comply with a testing procedure prescribed by a NPDWR, and operating under a variance or exemption.

till 1) Deposits of glacial drift laid down in place as the glacier melts, consisting of a heterogeneous mass of rock flour, clay, sand, pebbles, cobbles, and boulders intermingled in any proportion. 2) The agricultural cultivation of fields.

tillage Plowing, seedbed preparation, and cultivation practices.

tilth The physical condition of a soil as related to its ease of cultivation. Good tilth is associated with high, non-capillary porosity and stable, granular structure and low impedance to seedling emergence and root penetration.

time, distance, shielding The three main ways to minimize exposure to radiation.

time lag The time required for processes and control systems to respond to a signal or to reach a desired level.

timer A device for automatically starting or stopping a machine or other device at a given time.

time-weighted average The average value of a parameter (e.g., concentration of a chemical in air) that varies over time.

tissue A group of similar cells.

titer The concentration of a substance in a solution, or the strength of such a substance detected by titration. In the current context, the term is most likely to refer to antibody titer, which is a measure of the concentration of specific antibodies to selected microbes that are circulating in an individual's bloodstream.

titrate To titrate a sample, a chemical solution of known strength is added on a drop-by-drop basis until a certain color change, precipitate, or pH change in the sample is observed (end point).

titration The process of adding a chemical reagent of known strength in increments until completion of the reaction, as signaled by the end point. See titrate.

TNCWS Transient noncommunity water system.

toilet displacement device Object placed in a toilet tank to reduce the amount of water used per flush; for example, weighted plastic jugs filled with water or toilet dams that hold back a reservoir of water when the toilet is flushing. Considered effective only for fixtures using more than 3.5 gallons per flush. Also called toilet tank displacement device.

toilet flapper Valve in the toilet tank that controls flushing.

tolerance An organism's capacity to endure or adapt to (usually temporarily) infavorable environmental factors.

tolerant organism An organism exhibiting a capacity to survive relatively large environmental changes.

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.

too numerous to count The total number of bacterial colonies exceed 200 on a 47-mm diameter membrane filter used for coliform detection.

topography The physical features of a surface area including relative elevations and the position of natural and manmade features.

tornado A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena. They can occur anywhere in the world given the right conditions, especially after the landfall of hurricanes.

total dissolved phosphorous Total phosphorus content of material that will pass through a filter of specific size.

total dissolved solids (TDS) All of the dissolved solids in a water. TDS is measured on a sample of water that has passed through a very fine mesh filter to remove suspended solids. The water passing through the filter is evaporated and the residue represents the dissolved solids. Also see specific conductance.

total dynamic head When a pump is lifting or pumping water, the vertical distance (in feet) from the elevation of the energy grade line on the suction side of the pump to the elevation of the energy grade line on the discharge side of the pump.

total effective storage volume The volume available to store water in distribution reservoirs measured as the difference between the reservoir's overflow elevation and the minimum storage elevation. The minimum storage elevation is that elevation of water in the reservoir that can provide a minimum pressure of 20 psi at a flow as determined in 12 VAC 5-590-690 C, Virginia Waterworks Regulations, to the highest elevation served within that reservoir's service area under system-wide maximum daily water demand.

totalize A device that continuously sums up the flow into a plant in gallons or million gallons or some other unit of measurement.

totalizer A device or meter that continuously measures and calculates (adds) total flows in gallons, million gallons, cubic feet, or some other unit of volume measurement. Also called an integrator.

total Kjeldahl nitrogen (TKN) A test value that detects the sum of organic and ammonia forms of nitrogen.

total nitrogen The sum of all nitrogen forms.

total organic carbon The amount of carbon existing as part of organic compounds in a sample. Excludes inorganic forms of carbon, such as CO₂ and CaCO₃.

total particulate phosphorous Total phosphorous content of material retained on a filter of specific size.

total phosphorous The sum of all phosphorous forms.

total residual chlorine The amount of available chlorine remaining after a given contact time. The sum of the combined available residual chlorine and the free available residual chlorine. Also see residual chlorine.

total suspended particles A method of monitoring particulate matter by total weight.

total suspended solids (tss) A measure of the suspended solids in wastewater, effluent, or water bodies, determined by tests for “total suspended nonfilterable solids.”

total trihalomethanes (TTHMs) The sum of the concentration of the several trihalomethane compounds (trichloromethane[chloroform], dibromochloromethane, bromodichloromethane, and tribromomethane [bromoform]) rounded to two significant figures when expressed as mg/L.

touch screen Input device over the television or a special computer screen that is used to simplify user input and response. The user touches the screen rather than a keyboard, keypad, or mouse to control the output. Touch screens work best with menus or multiple-choice decision points, and also allow some simulation of hands-on training, i.e. pointing to parts of a body.

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

toxic A substance that is poisonous to an organism upon ingestion, absorption, or physical contact.

toxicant A harmful substance or agent that may injure an exposed organism.

toxic chemical Any chemical listed as "Toxic Chemicals Subject to Section 313 of the Emergency Planning and Community Right-to-Know Act of 1986."

toxic chemical release form Information form required of facilities that manufacture, process, or use (in quantities above a specific amount) chemicals listed under SARA Title III.

toxic chemical use substitution Replacing toxic chemicals with less harmful chemicals in industrial processes.

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

toxicity The quality or degree of being poisonous or harmful to plant, animal, or human life.

toxicity assessment Characterization of the toxicological properties and effects of a chemical, including all aspects of its absorption, metabolism, excretion, and mechanism of action, with special emphasis on establishment of dose-response characteristics.

toxicity testing Biological testing (usually with an invertebrate, fish, or small mammal) to determine the adverse effects of a compound or effluent.

toxicological profile An examination, summary, and interpretation of a hazardous substance to determine levels of exposure and associated health effects.

toxicology The science and study of poison control.

toxic pollutants Materials contaminating the environment that cause death, disease, and 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 Database of toxic releases in the United States compiled from SARA Title III section 313 reports.

toxic substance A chemical or mixture that may represent an unreasonable risk of injury to health or the environment.

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

toxins Proteins or conjugated protein substances which are lethal to other organisms. They are produced by some higher plants, certain animals, and pathogenic bacteria. The high molecular weight and antigenicity of toxins differentiates them from simple chemical poisons and vegetable alkaloids.

trace elements Chemical elements appearing in minute quantities in natural systems or media; may occasionally be concentrated by specific organisms. Nutrients, such as phosphorous, though in minute quantities are not usually called trace elements.

tracer tests Tests that measure the reduction over time of the concentration of a tracer as well as the arrival times of groundwater flow at a known point. Possible tracers include salt, radioactive isotopes, and fluorescent dyes.

trade winds The wind system, occupying most of the tropics, which are northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere.

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

transcription The first stage in the expression of a gene by means of genetic information being transmitted from the dna in the chromosomes to messenger rna.

transducer A device that senses some varying condition and converts it to an electrical signal for transmission to some other device (a receiver) for processing or decision making.

transect A line or belt through a community on which are indicated the important characteristics of the individuals of the species observed; sampling along a transect may be plotless or refer to specific plots.

transfer, water exchange of water among willing buyers and sellers.

transformation The physical, chemical, and biological processes by which a molecule of a chemical is altered to form a higher- or lower-molecular-weight chemical.

transient water system (TWS) A noncommunity 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 noncommunity water system (TNCWS).

translation The second stage in the expression of a gene by means of genetic information being transmitted from the mrna to the synthesis of protein.

transmission The direct or indirect (such as, waterborne) transfer of an infectious agent from a reservoir to a susceptible host.

transmission facilities See transmission lines.

transmission lines Pipelines that transport raw water from its source to a water treatment plant. After treatment, water is usually pumped into pipelines (transmission lines) that are connected to a distribution grid system.

transmission main A water main whose primary purpose is to move significant quantities of treated water among service areas.

transmission of infection Any mode or mechanism by which an infectious agent is spread through the environment or to another person.

transmission of infectious agents Any mechanism through which an infectious agent, such as a virus, is spread from a reservoir (or source) to a human being. Usually each type of infectious agent is ordinarily spread by only one or a few of the different mechanisms. There are several types of transmission mechanisms:

a) Direct transmission: This type of transmission is, at base, immediate. The transfer of the infectious agent is, as the name implies, directly into the body. Different infectious agents may get into the body using different routes. Some routes by which infectious diseases are spread directly include personal contact, such as touching, biting, kissing or sexual intercourse. In these cases the agent enters the body through the skin, mouth, an open cut or sore or sexual organs. Infectious agents may spread by tiny droplets of spray directly into the conjunctiva or the mucus membranes of the eye, nose or mouth during sneezing, coughing, spitting, singing or talking (although usually this type of spread is limited to about within one meter's distance.) This is called droplet spread.

b) Indirect transmission: Indirect transmission may happen in any of several ways.

Vehicle-borne transmission:

In this situation, a vehicle—that is, an inanimate object or material called in scientific terms a fomite, becomes contaminated with the infectious agent. The agent, such as a virus, may or may not have multiplied or developed in or on the vehicle. The vehicle contacts the person's body. It may be ingested (eaten or drunk), touch the skin, or be introduced internally during surgery or medical treatment. Examples of vehicles that can transmit diseases include cooking or eating utensils, bedding or clothing, toys, surgical or medical instruments (like catheters) or dressings. Water, food, drinks (like milk) and biological products like blood, serum, plasma, tissues or organs can also be vehicles.

Vector-borne transmission:

When researchers talk about vectors, often they are talking about insects, which as a group of invertebrate animals carry a host of different infectious agents. (However, a vector can be any living creature that transmits an infectious agent to humans.)

Vectors may mechanically spread the infectious agent, such as a virus or parasite. In this scenario the vector—for instance a mosquito— contaminates its feet or proboscis ("nose") with the infectious agent, or the agent passes through its gastrointestinal tract. The agent is transmitted from the vector when it bites or touches a person. In the case of an insect, the infectious agent may be injected with the insect's salivary fluid when it bites. Or the insect may regurgitate material or deposit feces on the skin, which then enter a person's body, typically through a bite wound or skin that has been broken by scratching or rubbing.

In the case of some infectious agents, vectors are only capable of transmitting the disease during a certain time period. In these situations, vectors play host to the agent. The agent needs the host to develop and mature or to reproduce (multiply) or both (called cyclopropagative). Once the agent is within the vector animal, an incubation period follows during which the agent grows or reproduces or both, depending on the type of agent. Only after this phase is over does the vector become infective. That is, only then can it transmit an agent that is capable of causing disease in the person.

c) Airborne transmission: In this type of transmission, infective agents are spread as aerosols, and usually enter a person through the respiratory tract. Aerosols are tiny particles, consisting in part or completely of the infectious agent itself, that become suspended in the air. These particles may remain suspended in the air for long periods of time, and some retain their ability to cause disease, while others degenerate due to the effects of sunlight and dryness. When a person breathes in these particles, they become infected with the agent—especially in the alveoli of the lungs.

transmissivity The rate at which water of a prevailing density and viscosity is transmitted through a unit width of an aquifer or confining bed under a unit hydraulic gradient. It is a function of the properties of the liquid, the porous media, and the thickness of the porous media.

transpiration Process by which water that is absorbed by plants, usually through the roots, is evaporated into the atmosphere from the plant surface. See also evaporation and evapotranspiration.

transponder The electronic equipment on a satellite that receives signals from an uplink, converts signals to a new frequency, amplifies the signal, and sends it back to earth. Satellites are usually equipped with 12 to 14 transponders.

transportation control measures Steps taken by a locality to improve air quality by reducing or changing he flow of traffic, e/g., public transit, carpools, HOV lanes, etc.

trash Material considered worthless or offensive that is thrown away. Generally defined as dry waste material, but in common usage it is a synonym for garbage, rubbish, or refuse.

trash-to-energy plan Burning trash to produce energy.

traveling bridge A support mechanism that is propelled across or along the length of a treatment basin and transports equipment that collects sludge, provides backwashing to a filter, or aerates sequentially while the rest of the treatment process continues.

treatability studies Tests of wastewater treatment technologies on unusual or highly contaminated waste streams and potential cleanup technologies conducted in a laboratory. See bench-scale tests.

treated regulated medical waste Medical waste treated to substantially reduce or eliminate its pathogenicity, but has not yet been destroyed.

treated wastewater Wastewater that has been subjected to one or more physical, chemical, and biological processes to reduce the risk of health hazard and pollution of the environment resulting from the discharge of the treated wastewater.

treated water Water treated to meter drinking water standards.

treatment 1) Any method, technique, or process designed to remove solids and/or pollutants from solid waste, wastestreams, effluents, and air emissions. 2) Methods used to change the biological character or composition of any regulated medical waste so as to substantially reduce or eliminate its potential for causing disease.

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

treatment, storage, and disposal facility Site where a hazardous substance is treated, stored, or disposed of.

treatment technique requirement A requirement which specifies for a contaminant a specific treatment technique or techniques demonstrated to the satisfaction of the Commissioner to lead to a reduction in the level of such contaminant sufficient to comply with provisions of the regulations.

tremie A device used to place concrete or grout under water.

trend A long-term movement or change in frequency, usually upwards or downwards.

trial burn An incinerator test in which emissions are monitored for the presence of specific organic compounds, particulates, and hydrogen chloride.

trichloroethane (TCE) An organic chemical used as a cleaning solvent that causes adverse health effects in drinking water supplies.

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

trickling filter A contactor using support medium for bacterial growth, usually a bed of rocks or stone. The wastewater passes through the bed so that the media biofilm oxidize the organic wastes.

trickling filter media The solid material in a trickling filter that provides a surface for a biological film of microorganisms. Crushed stone is the most commonly used media, but plastics are gaining popularity.

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

trigger level A specified concentration of a contaminant above which a system must increase monitoring.

trihalomethane One of a family of organic compounds, named as derivatives of methane. THMs are generally the by-product from chlorination of drinking water that contains naturally occurring fulvic and humic organic matter. The resulting compounds, THMs, are suspected of causing cancer.

trilinear diagram A method of graphically plotting the chemical composition of the major anions and cations of a water sample.

tripton The nonliving component of the seston; suspended, nonliving matter in a body of water.

trophic Of or related to nourishment or feeding. See eutrophic.

tropical cyclone A general term for all cyclone circulations originating over tropical waters. Its characteristics include a warm-core, non-frontal pressure system of synoptic scale that originates over the tropical or subtropical waters and has a definite organized surface. Used to define wind circulations rotating around an atmosphere which include tropical depressions, tropical storms, and hurricanes. The strongest winds of this cyclone are near the Earth's center.

tropical depression (TD) A tropical cyclone in which the maximum sustained surface winds (1 minute average) are 38 miles per hour (33 knots) or less. Characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave, which has continued to organize. At this point, it gets a cyclone number, starting with "TD01" at the beginning of each storm season.

tropical disturbance A discrete system of clouds, showers, and thunderstorms (organized convection) that originate in the tropics. Generally 100 to 300 miles in diameter and originating in the tropics or subtropics, disturbances have a nonfrontal migratory character, and maintain their identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field. An upper level of low pressure causes this to occur. Approximately 100 of these types of events occur annually during hurricane season.

tropical storm (TS) A tropical cyclone in which the maximum sustained surface wind speed (1 minute average) is within the range of 39 to 73 mph (34 to 63 knots). At this point, the system is given a name to identify and track it. In the Atlantic/Caribbean/Gulf of Mexico basin, the names start with "A" each season.

tropical storm warning A warning issued by the National Hurricane Center for tropical storm conditions including possible sustained winds within the range 39 to 73 mph (34 to 63 knots) which are expected in a specified coastal area within 24 hours or less.

tropical storm watch An announcement issued by the National Hurricane Center for specific areas that a tropical storm or a forecast of tropical storm conditions poses a possible threat to coastal areas generally within 36 hours. A tropical storm watch normally should not be issued if the system is forecast to attain hurricane strength.

tropical wave Another name for an easterly wave, it is an area of relatively low pressure (trough) moving westward through the trade wind easterlies. Generally, it is associated with extensive cloudiness and showers, and may be associated with possible tropical cyclone development.

tropics The region of the earth located between the Tropic of Cancer, at 23.5 degrees North latitude, and the Tropic of Capricorn, at 23.5 degrees South latitude. It encompasses the equatorial region, an area of high temperatures and considerable precipitation during part of the year.

troposphere The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height (except, perhaps, in thin layers - see inversion, cap), vertical wind motion, appreciable water vapor content, and sensible weather (clouds, rain, etc.).

trust fund, CERCLA A fund set up under the CERCLA to help pay for cleanup of hazardous waste sites and for legal action to force those responsible for the sites to clean them up.

t-suppressor cells T-lymphocytes with specific capacity to inhibit t-helper cell function.

tube settler A device that uses bundles of small bore (2 to 3 inches or 50 to 70 millimeters) tubes installed on an incline as an aid to sedimentation. The tubes may come in a variety of shapes including circular and rectangular. As water rises within the tubes, settling solids fall to the tube surface. As the sludge, from the settled solids, in the tube gains weight, it moves down the tubes and settles to the bottom of the basin for removal by conventional sludge collection means. Tube settlers are sometimes installed in sedimentation basins and clarifiers to improve particle removal.

tubercle A protective crust of corrosion products (rust) which builds up over a pit caused by the loss of metal due to corrosion.

tuberculation The development of formation of small mounds of corrosion products (rust) on the inside of iron pipe. These mounds (tubercules) increase the roughness of the inside of the pipe thus increasing resistance to water flow (decreases the C factor).

tumor incidence Fraction of animals having a tumor of a certain type.

tundra Arctic, subartic, or high alpine land, devoid of trees, with mosses and sedges dominant. In the Arctic, tundra is underlain by permafrost, the upper layer of which thaws in the summer.

turbid Having a cloudy or muddy appearance.

turbidimeter A device that measures the amount of suspended solids in a liquid.

turbidity The cloudy appearance of water caused by the light scattering effect 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.

turbulent A random, irregular, highly mixed flow of fluid.

turnover 1) Mixing of a water body from top to bottom, ordinarily in the spring and fall, resulting from wind action on uniformly heated (or at least uniformly dense water); also called overturn. 2) The reciprocal of residence time, an aspect of the stability or persistance of a component.

two-way video/two-way audio Interactive video in which all sites are in visual contact with one another. Some form of audioconferencing is used for real-time verbal interaction.

TWS Transient water system.

two-stage filters Two filters are used with the effluent from the first filter becoming the influent to the second filter, either directly or with a clarifier in between.